Once again, an earlier than expected finish to a Group A match which means we are wrapping up this live text ridiculously early. As a positive, it will give us more time to revise for tomorrow afternoon's clash between Roger Federer and Kei Nishikori, which is live on BBC Two and this website. Good, eh? See you tomorrow for more tennis fun.
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Novak Djokovic is chatting outside his locker room to a member of One Direction, encouraging him to settle down and become a father. Djokovic will suffer the ire of 'Directioners' if he's not careful. It isn't Harry Styles, nor the Doncaster Rovers fan. He has an Irish lilt so Niall Horan perhaps?
Post update
If value for money is weighed against minutes on court, today's visitors to the 02 have not had much for their pound as the two singles matches lasted a combined total of one hour and 54 minutes, but at least they were treated to an eye-wateringly high standard of tennis from Stan Wawrinka and Novak Djokovic.
Get involved #bbctennis
BBC SportCopyright: BBC Sport
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Djokovic is still on court, adding his autograph to Serbian flags and giant tennis balls. There's also a number of selfies, too. Girls, women, grown men are calling out his name. He's already spent more minutes on court signing autographs than he did winning the first set.
Post update
It will come as no surprise to hear that Djokovic won't be visiting any of the Big Smoke's watering holes this week.
"I try to spend as much time outdoors as possible," says Djokovic when asked how he will spend his time in London. "I try to spend some time in the park with my little dog. Unfortunately, my family - my wife and baby - aren't here. I'm not going to the bar or pub, if that's what you meant."
Asked how he manages to return with such distinction, Djokovic replies: "Most of the time, it's obviously the practice that makes it perfect and a lot of hours spent on court. Obviously, it's one of the elements in the game I can rely on and it's very useful when you're playing a big server."
Post update
A fresh-looking Novak Djokovic says: "It's been a great, great match. I haven't played here for 12 months, it's great to be back. It was a great performance.
"Marin as a US Open winner has a lot of confidence, but I managed to neutralise his serve and get a lot of ball back in play and that was part of the tactic."
Game, set and match
Djokovic 6-1 6-1 Cilic
Djokovic collects three match points and needs just the one, Cilic failing on the return to gift Djokovic a hold to love and, more importantly, the match. That was 56 minutes of textbook tennis from Djokoivc, who extends his unbeaten run indoors to 28 matches.
Post update
David Law
BBC Radio 5 live sports extra commentator
AFPCopyright: AFP
"It's turning into a nightmare for Marin Cilic, the US Open champion, here. This is pretty much over. It's been a rough day for the organisers."
Post update
*Djokovic 6-1 5-1 Cilic
Canny from Cilic, beating Djokovic with a backhand crosscourt winner. Too little too late? Perhaps, but at least he's offering some resistance and is refusing to sink like a badly cooked soufflé. Ah. Hang on. A series of Cilic errors presents Djokovic with a chance and the Serb has a break point. How does Cilic respond? He balloons a forehand beyond the baseline. Djokovic to serve for the match.
Post update
David Law
BBC Radio 5 live sports extra commentator
Getty ImagesCopyright: Getty Images
"This is just cruise control for Novak Djokovic. He'll be disgruntled that he's lost a game in this set."
Post update
Djokovic 6-1 4-1 Cilic*
An overhead from Djokovic eases him to a 40-0 lead and Cilic is offering little resistance. The US Open champion slaps a return into the tape, and Djokovic can now sniff the finishing line.
Over and out?
ATP World Tour Finals 2014 stat: That 27-minute set is the third straight 6-1 set after Tomas Berdych's thrashing by Stan Wawrinka earlier.
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Piers Newbery
BBC Sport at the O2 Arena in London
"While the Djokovic masterclass has been something to behold for the opening 37 minutes, it comes as something of a relief to the paying punters when Cilic finds his US Open form to get back on serve.
"The Croatia football squad is in the crowd ahead of their friendly against Argentina at Upton Park and their trip to the O2 wasn't looking like much of a morale boost. Mollie from The Saturdays, on the other hand, appears to be having a lovely time."
Djokovic breaks
*Djokovic 6-1 3-1 Cilic
Cilic must play without fear in this game. He opens with a booming ace and another unreturnable serve out wide puts the Croat 30-0 ahead. Cilic determined to prove he can't be bullied, but from 30-0 to 30-30 we go, Djokovic slicing his way back into contention.
Cilic throwing his biggest punches but Djokovic standing firm. From 30-30 to deuce to break point thanks to a series of punishing forehands from Djokovic. Cilic's serve gets him out of danger, saving one break point and then another... third break point Djokovic... Cilic caves in, going long and Djokovic breaks for a fifth time in this match.
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David Cornelius: "May be early days at O2, but initial results evidence the gulf between 1-4 and 5-8 in the current rankings."
Cilic breaks
Djokovic 6-1 2-1 Cilic*
AFPCopyright: AFP
Raucous applause for the underdog as he collects three points on the trot. Triple break points! And the revived Croat lasers a forehand winner down the line to nick a point and the game!
"Even if Roger Federer was out here right now he would be struggling. Djokovic is in such good form."
APCopyright: AP
Djokovic breaks
*Djokovic 6-1 2-0 Cilic
Cilic clubs a series of muscly forehands, ones which would have floored many an opponent, but the rubber-legged Djokovic simply slides along the baseline fetching and retrieving before his defence does eventually falter. A well-constructed point from Cilic, but it seems as if he needs to hit three winners to win a point.
Djokovic hauls Cilic to 30-30. On the next point, Cilic works Djokovic from side to side and still the Serb won't give in. Eventually, Cilic's forehand clips the net and bounces wide, presenting Djokovic with a break point. A shake of the head from Goran Ivanisevic. It could soon be game over because Cilic falters once more and Djokovic has another break.
World number one and Bond villain look-a-like Djokovic emerges from the moody O2 smokeImage caption: World number one and Bond villain look-a-like Djokovic emerges from the moody O2 smoke
"Ominous signs for Cilic tonight. There's so many different aspects to Djokovic's game. We spoke about Cilic going after him, but Djokovic defends so well, it's almost like the hunter becomes the hunted."
Post update
Djokovic 6-1 1-0 Cilic*
There's no need to be a tennis aficionado to know that Cilic must improve in this set - He had just a 45% success rate on his first serve in the opening set. Business as usual for Wimbledon champion Djokovic, who says becoming a father has given him added strength. On this form, other players on the tour will be hoping Djokovic retires before deciding to have a second child. A hold to 15 for the two-time defending champ.
Game and first set
*Djokovic 6-1 Cilic
Wonderful from Djokovic, pinning Cilic back with a down-the-line forehand before killing the Croat's hopes with a feather-soft drop shot on the other side of the court. Exhibition stuff from the world number one who easily collects a set point. Cilic is getting a beating, but at least he has a sense of humour, firing down his first ace of the match to save set point. A second set point quickly comes Djokovic's way, though, and a drilled Cilic backhand into the tape finishes off the set.
"It's tough to see right now what Cilic can do to get Djokovic out of his rhythm. Everything he throws at him, Djokovic is knocking it right back."
Post update
Djokovic 5-1 Cilic*
There was talk before this match that it was going be a far more competitive contest than the one we witnessed between Stan Wawrinka and Tomas Berdych this afternoon, but that seems unlikely because Djokovic is now firing on all cylinders. The Serb has won 22 of the last 29 points and secures another impressive hold to love.
Djokovic breaks
*Djokovic 4-1 Cilic
Cilic starts with a double fault and by the time he screws a forehand into the tramlines he finds himself 40-0 down. Djokovic raises the intensity, pouncing with a speed of a cobra's tongue to break the Cilic serve for the second time in this match.
"Marin Cilic is an extremely personable, intelligent, charming man, who seems to be coping with all the extra razzmatazz quite well after winning the US Open.
"Then you've got these other names Milos Raonic, Gregor Dimitrov, who have still got a way to go, but the way Kei Nishikori played this year in reaching the US Open final shows there are eight or nine potential champions for Melbourne [Australian Open]. We've not had that for years."
Post update
Djokovic 3-1 Cilic*
Djokovic yet to unleash his best tennis, but he has not had to. He's simply slicing and dicing against a Cilic who is lacking accuracy. The US Open champion sprints towards the tape only to push his volley wide, slipping 40-15 behind - but he adjusts his footwork nicely following a net cord to drive a deep volley onto the line. But Djokovic's defence is impenetrable, the Serb gliding around the court before completing a hold to 30 with a backhand winner down the line.
Djokovic breaks
* Djokovic 2-1 Cilic
Here's a Cilic stat for you: he's hit 733 aces this season (his previous best was 558). No aces so far in this game, but the Croat does battle back from 0-30 to 30-30. But having dug himself out of a hole, Cilic jumps into a deep well without a ladder, overcooking a forehand and dragging another wide to present Djokovic with an early break.
"We've seen so far this week that the court is quite slow, quite dead. So when players come to the net and hit those underspin volleys, they're not winning as many points as they would hope for."
Post update
Djokovic 1-1 Cilic*
BBC SportCopyright: BBC Sport
At 30-30 Djokovic seems on edge, but the world number one comes up with a serve which has more swing and spin than a Shane Warne special. Uncharacteristic errors from Djokovic, ending a lengthy rally with an overcooked forehand. So far, Cilic standing firm.
At advantage, Djokovic has the chance to dismiss the Cilic threat but he messes up an overhead - "Oohs" of disbelief from the 17.000 crowd - but the Serb wraps up the game, finishing with a cool ace.
Post update
*Djokovic 0-1 Cilic
The warm-up is over, we're now ready for some serious stuff. Cilic's coach, Goran Ivanisevic, watching on from the sidelines and he'll be content with how his pupil has started. Djokovic drags a forehand into the tramlines to allow Cilic to breeze to 40-0. The Serb yet to adjust his radar. Another error from the defending champion, and Cilic holds to love.
Try, try and try again
But no matter how hard he tries, Cilic has yet to come up with a master plan to defeat Djokovic, with the top seed winning all 10 previous matches between the pair. The Croat has got close this year, however, forcing Djokovic to come back from two sets to one down at Wimbledon this year to win that quarter-final clash.
"It's not a surprise to see Cilic here at the O2, he's got the game to be in the top eight.
"His coach Goran Ivanisevic has changed the rhythm on his serve, a lower ball toss and a quicker technique. It seems to have added 5mph on to the serve and makes it harder for the returner to read. I think it had really bedded in by the US hard court season and we saw the results."
Gate-crashing the Grand Slam hierarchy
Getty ImagesCopyright: Getty Images
True fact: Marin Cilic's US Open victory completes a Grand Slam year that has seen eight different winners of the singles titles across the men's and women's games, for the first time since 1998.
Not so true fact: Marin Cilic treats that precious US Open trophy to a luxurious limousine ride every single day.
Cilic's winning wind
Marin Cilic tells BBC Sport how winning the US Open has benefited him:
"There has been much more attention, media attention - those are all positive parts of course because when you are a tennis player you want to achieve big things. I'm expecting much more of myself week after week to build up my game.
"It showed me proof that all the things we did in the past for New York - we did them right. There are no doubts in my head if we are doing correct things or not, that's going to give me a bigger wind on my back."
Post update
David Law
BBC Radio 5 live sports extra commentator
"Cilic looks a little bit nervous. Perhaps the reason why is this man, Novak Djokovic, who looks like he's walking out into his front room."
Cilic making giant strides
BBCCopyright: BBC
What a year it's been for Marin Cilic. The 26-year-old Croat is making his ATP World Finals debut after climbing from 37 to nine in the world thanks to his first Grand Slam crown and three other titles. Impressively, he was the first player outside the Top 10 to win the US Open since Pete Sampras, ranked 17th in the world, did so in 2002.
"My worst match of the whole season," is how the Czech described the one-sided contest. You can watch Wawrinka's eye-catching shots in that match on this website.
All Berdych could do after the match was tweet a picture of a monkey hiding his eyes. Don't worry, Tomas, we've all been at the end of a straight-sets mullering, though perhaps not in front of 17,000 people.
BBCCopyright: BBC
Post update
Piers Newbery
BBC Sport at the O2 Arena in London
"It looks like a full house and the evening crowd has been settled in for some time to greet the world number one and US Open champion, with one huge graphic bearing down from the screen above: Djokovic 10-0 Cilic. Unbeaten on this court since 2011 and brimming with confidence, it's hard to find anyone who doesn't think the two-time champion will make it a hat-trick on Sunday."
Post update
"I've played good tennis in the last couple of months and this is another opportunity to play well," says a relaxed Marin Cilic as he is stopped coming out of his locker room and placed in front of a TV camera.
Novak Djokovic is the next before the inquisitor and the Serb says "it feels great" to be back competing at the O2. "I'm sure he has plenty of confidence and is motivated to play every match likes it's the last one of the season."
Both players in the corridor, waiting to be called into action; Djokovic loosening up his limbs as he waits, Cilic joking with suited officials.
Get involved via #bbctennis, BBC Sport Facebook page or text 81111
But while these two might make professional sport and parenthood look like easy bedfellows - we want to hear about real life, away from the private jets, army of nannies and gold-plated dummies.
What are your biggest parental sporting disasters?
Andy Murray said his journey via boat to the O2 "wakes him up" - and Stan Warwinka posted a picture of himself on the Thames as he meandered towards this afternoon's crushing of Tomas Berdych.
Now Novak's joined in - snapping a picture of the Tower of London on his aptly-stencilled transport, tweeting: "On the way to the The O2 Arena. 'The time has come'. Idemo!"
He'd make a good PR man would Djokovic.
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Hello! Welcome to live text commentary of Novak Djokovic's Group A clash against Marin Cilic in the ATP World Tour Finals.
Can Novak Djokovic make it three ATP World Finals titles in a row? His bid to emulate Ivan Lendl's hat-trick of successive titles begins this evening against US Open champion Marin Cilic. They're about to get down to business.
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Getty ImagesCopyright: Getty Images
Since October, the world's number one tennis player has been changing nappies, shopping for buggies and singing Serbian nursery rhymes*. Recently married and a father of a baby boy - Novak Djokovic has it all. But for the next week the 27-year-old needs to prove he's still the daddy on the court.
(*I'm not entirely sure he's been singing nursery rhymes - but who doesn't?)
Live Reporting
By Aimee Lewis
All times stated are UK
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Once again, an earlier than expected finish to a Group A match which means we are wrapping up this live text ridiculously early. As a positive, it will give us more time to revise for tomorrow afternoon's clash between Roger Federer and Kei Nishikori, which is live on BBC Two and this website. Good, eh? See you tomorrow for more tennis fun.
Post update
Novak Djokovic is chatting outside his locker room to a member of One Direction, encouraging him to settle down and become a father. Djokovic will suffer the ire of 'Directioners' if he's not careful. It isn't Harry Styles, nor the Doncaster Rovers fan. He has an Irish lilt so Niall Horan perhaps?
Post update
If value for money is weighed against minutes on court, today's visitors to the 02 have not had much for their pound as the two singles matches lasted a combined total of one hour and 54 minutes, but at least they were treated to an eye-wateringly high standard of tennis from Stan Wawrinka and Novak Djokovic.
Get involved #bbctennis
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Djokovic is still on court, adding his autograph to Serbian flags and giant tennis balls. There's also a number of selfies, too. Girls, women, grown men are calling out his name. He's already spent more minutes on court signing autographs than he did winning the first set.
Post update
It will come as no surprise to hear that Djokovic won't be visiting any of the Big Smoke's watering holes this week.
"I try to spend as much time outdoors as possible," says Djokovic when asked how he will spend his time in London. "I try to spend some time in the park with my little dog. Unfortunately, my family - my wife and baby - aren't here. I'm not going to the bar or pub, if that's what you meant."
Post update
Asked how he manages to return with such distinction, Djokovic replies: "Most of the time, it's obviously the practice that makes it perfect and a lot of hours spent on court. Obviously, it's one of the elements in the game I can rely on and it's very useful when you're playing a big server."
Post update
A fresh-looking Novak Djokovic says: "It's been a great, great match. I haven't played here for 12 months, it's great to be back. It was a great performance.
"Marin as a US Open winner has a lot of confidence, but I managed to neutralise his serve and get a lot of ball back in play and that was part of the tactic."
Game, set and match
Djokovic 6-1 6-1 Cilic
Djokovic collects three match points and needs just the one, Cilic failing on the return to gift Djokovic a hold to love and, more importantly, the match. That was 56 minutes of textbook tennis from Djokoivc, who extends his unbeaten run indoors to 28 matches.
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David Law
BBC Radio 5 live sports extra commentator
"It's turning into a nightmare for Marin Cilic, the US Open champion, here. This is pretty much over. It's been a rough day for the organisers."
Post update
*Djokovic 6-1 5-1 Cilic
Canny from Cilic, beating Djokovic with a backhand crosscourt winner. Too little too late? Perhaps, but at least he's offering some resistance and is refusing to sink like a badly cooked soufflé. Ah. Hang on. A series of Cilic errors presents Djokovic with a chance and the Serb has a break point. How does Cilic respond? He balloons a forehand beyond the baseline. Djokovic to serve for the match.
Post update
David Law
BBC Radio 5 live sports extra commentator
"This is just cruise control for Novak Djokovic. He'll be disgruntled that he's lost a game in this set."
Post update
Djokovic 6-1 4-1 Cilic*
An overhead from Djokovic eases him to a 40-0 lead and Cilic is offering little resistance. The US Open champion slaps a return into the tape, and Djokovic can now sniff the finishing line.
Over and out?
ATP World Tour Finals 2014 stat: That 27-minute set is the third straight 6-1 set after Tomas Berdych's thrashing by Stan Wawrinka earlier.
Post update
Piers Newbery
BBC Sport at the O2 Arena in London
"While the Djokovic masterclass has been something to behold for the opening 37 minutes, it comes as something of a relief to the paying punters when Cilic finds his US Open form to get back on serve.
"The Croatia football squad is in the crowd ahead of their friendly against Argentina at Upton Park and their trip to the O2 wasn't looking like much of a morale boost. Mollie from The Saturdays, on the other hand, appears to be having a lovely time."
Djokovic breaks
*Djokovic 6-1 3-1 Cilic
Cilic must play without fear in this game. He opens with a booming ace and another unreturnable serve out wide puts the Croat 30-0 ahead. Cilic determined to prove he can't be bullied, but from 30-0 to 30-30 we go, Djokovic slicing his way back into contention.
Cilic throwing his biggest punches but Djokovic standing firm. From 30-30 to deuce to break point thanks to a series of punishing forehands from Djokovic. Cilic's serve gets him out of danger, saving one break point and then another... third break point Djokovic... Cilic caves in, going long and Djokovic breaks for a fifth time in this match.
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David Cornelius: "May be early days at O2, but initial results evidence the gulf between 1-4 and 5-8 in the current rankings."
Cilic breaks
Djokovic 6-1 2-1 Cilic*
Raucous applause for the underdog as he collects three points on the trot. Triple break points! And the revived Croat lasers a forehand winner down the line to nick a point and the game!
Post update
Colin Fleming
British doubles player on BBC Radio 5 live sports extra
"Even if Roger Federer was out here right now he would be struggling. Djokovic is in such good form."
Djokovic breaks
*Djokovic 6-1 2-0 Cilic
Cilic clubs a series of muscly forehands, ones which would have floored many an opponent, but the rubber-legged Djokovic simply slides along the baseline fetching and retrieving before his defence does eventually falter. A well-constructed point from Cilic, but it seems as if he needs to hit three winners to win a point.
Djokovic hauls Cilic to 30-30. On the next point, Cilic works Djokovic from side to side and still the Serb won't give in. Eventually, Cilic's forehand clips the net and bounces wide, presenting Djokovic with a break point. A shake of the head from Goran Ivanisevic. It could soon be game over because Cilic falters once more and Djokovic has another break.
Post update
Colin Fleming
British doubles player on BBC Radio 5 live sports extra
"Ominous signs for Cilic tonight. There's so many different aspects to Djokovic's game. We spoke about Cilic going after him, but Djokovic defends so well, it's almost like the hunter becomes the hunted."
Post update
Djokovic 6-1 1-0 Cilic*
There's no need to be a tennis aficionado to know that Cilic must improve in this set - He had just a 45% success rate on his first serve in the opening set. Business as usual for Wimbledon champion Djokovic, who says becoming a father has given him added strength. On this form, other players on the tour will be hoping Djokovic retires before deciding to have a second child. A hold to 15 for the two-time defending champ.
Game and first set
*Djokovic 6-1 Cilic
Wonderful from Djokovic, pinning Cilic back with a down-the-line forehand before killing the Croat's hopes with a feather-soft drop shot on the other side of the court. Exhibition stuff from the world number one who easily collects a set point. Cilic is getting a beating, but at least he has a sense of humour, firing down his first ace of the match to save set point. A second set point quickly comes Djokovic's way, though, and a drilled Cilic backhand into the tape finishes off the set.
Post update
Colin Fleming
British doubles player on BBC Radio 5 live sports extra
"It's tough to see right now what Cilic can do to get Djokovic out of his rhythm. Everything he throws at him, Djokovic is knocking it right back."
Post update
Djokovic 5-1 Cilic*
There was talk before this match that it was going be a far more competitive contest than the one we witnessed between Stan Wawrinka and Tomas Berdych this afternoon, but that seems unlikely because Djokovic is now firing on all cylinders. The Serb has won 22 of the last 29 points and secures another impressive hold to love.
Djokovic breaks
*Djokovic 4-1 Cilic
Cilic starts with a double fault and by the time he screws a forehand into the tramlines he finds himself 40-0 down. Djokovic raises the intensity, pouncing with a speed of a cobra's tongue to break the Cilic serve for the second time in this match.
Post update
Russell Fuller
BBC tennis correspondent on Radio 5 live sports extra
"Marin Cilic is an extremely personable, intelligent, charming man, who seems to be coping with all the extra razzmatazz quite well after winning the US Open.
"Then you've got these other names Milos Raonic, Gregor Dimitrov, who have still got a way to go, but the way Kei Nishikori played this year in reaching the US Open final shows there are eight or nine potential champions for Melbourne [Australian Open]. We've not had that for years."
Post update
Djokovic 3-1 Cilic*
Djokovic yet to unleash his best tennis, but he has not had to. He's simply slicing and dicing against a Cilic who is lacking accuracy. The US Open champion sprints towards the tape only to push his volley wide, slipping 40-15 behind - but he adjusts his footwork nicely following a net cord to drive a deep volley onto the line. But Djokovic's defence is impenetrable, the Serb gliding around the court before completing a hold to 30 with a backhand winner down the line.
Djokovic breaks
* Djokovic 2-1 Cilic
Here's a Cilic stat for you: he's hit 733 aces this season (his previous best was 558). No aces so far in this game, but the Croat does battle back from 0-30 to 30-30. But having dug himself out of a hole, Cilic jumps into a deep well without a ladder, overcooking a forehand and dragging another wide to present Djokovic with an early break.
Post update
Colin Fleming
British doubles player on BBC Radio 5 live sports extra
"We've seen so far this week that the court is quite slow, quite dead. So when players come to the net and hit those underspin volleys, they're not winning as many points as they would hope for."
Post update
Djokovic 1-1 Cilic*
At 30-30 Djokovic seems on edge, but the world number one comes up with a serve which has more swing and spin than a Shane Warne special. Uncharacteristic errors from Djokovic, ending a lengthy rally with an overcooked forehand. So far, Cilic standing firm.
At advantage, Djokovic has the chance to dismiss the Cilic threat but he messes up an overhead - "Oohs" of disbelief from the 17.000 crowd - but the Serb wraps up the game, finishing with a cool ace.
Post update
*Djokovic 0-1 Cilic
The warm-up is over, we're now ready for some serious stuff. Cilic's coach, Goran Ivanisevic, watching on from the sidelines and he'll be content with how his pupil has started. Djokovic drags a forehand into the tramlines to allow Cilic to breeze to 40-0. The Serb yet to adjust his radar. Another error from the defending champion, and Cilic holds to love.
Try, try and try again
But no matter how hard he tries, Cilic has yet to come up with a master plan to defeat Djokovic, with the top seed winning all 10 previous matches between the pair. The Croat has got close this year, however, forcing Djokovic to come back from two sets to one down at Wimbledon this year to win that quarter-final clash.
Post update
Colin Fleming
British doubles player on BBC Radio 5 live sports extra
"It's not a surprise to see Cilic here at the O2, he's got the game to be in the top eight.
"His coach Goran Ivanisevic has changed the rhythm on his serve, a lower ball toss and a quicker technique. It seems to have added 5mph on to the serve and makes it harder for the returner to read. I think it had really bedded in by the US hard court season and we saw the results."
Gate-crashing the Grand Slam hierarchy
True fact: Marin Cilic's US Open victory completes a Grand Slam year that has seen eight different winners of the singles titles across the men's and women's games, for the first time since 1998.
Not so true fact: Marin Cilic treats that precious US Open trophy to a luxurious limousine ride every single day.
Cilic's winning wind
Marin Cilic tells BBC Sport how winning the US Open has benefited him:
"There has been much more attention, media attention - those are all positive parts of course because when you are a tennis player you want to achieve big things. I'm expecting much more of myself week after week to build up my game.
"It showed me proof that all the things we did in the past for New York - we did them right. There are no doubts in my head if we are doing correct things or not, that's going to give me a bigger wind on my back."
Post update
David Law
BBC Radio 5 live sports extra commentator
"Cilic looks a little bit nervous. Perhaps the reason why is this man, Novak Djokovic, who looks like he's walking out into his front room."
Cilic making giant strides
What a year it's been for Marin Cilic. The 26-year-old Croat is making his ATP World Finals debut after climbing from 37 to nine in the world thanks to his first Grand Slam crown and three other titles. Impressively, he was the first player outside the Top 10 to win the US Open since Pete Sampras, ranked 17th in the world, did so in 2002.
Wawrinka crushes Berdych
There's already been a match in the wonderful 02 today and there are plenty of words to describe Tomas Berdych's 58-minute 6-1 6-1 defeat by Stan Wawrinka: Shend? Banjaxed? Clobbered? Thrashed?
"My worst match of the whole season," is how the Czech described the one-sided contest. You can watch Wawrinka's eye-catching shots in that match on this website.
All Berdych could do after the match was tweet a picture of a monkey hiding his eyes. Don't worry, Tomas, we've all been at the end of a straight-sets mullering, though perhaps not in front of 17,000 people.
Post update
Piers Newbery
BBC Sport at the O2 Arena in London
"It looks like a full house and the evening crowd has been settled in for some time to greet the world number one and US Open champion, with one huge graphic bearing down from the screen above: Djokovic 10-0 Cilic. Unbeaten on this court since 2011 and brimming with confidence, it's hard to find anyone who doesn't think the two-time champion will make it a hat-trick on Sunday."
Post update
"I've played good tennis in the last couple of months and this is another opportunity to play well," says a relaxed Marin Cilic as he is stopped coming out of his locker room and placed in front of a TV camera.
Novak Djokovic is the next before the inquisitor and the Serb says "it feels great" to be back competing at the O2. "I'm sure he has plenty of confidence and is motivated to play every match likes it's the last one of the season."
Both players in the corridor, waiting to be called into action; Djokovic loosening up his limbs as he waits, Cilic joking with suited officials.
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Superhuman Novak Djokovic is yet to drop a set since becoming a father, winning the Paris Masters last week and picking up some advice from father-of-four Roger Federer along the way.
But while these two might make professional sport and parenthood look like easy bedfellows - we want to hear about real life, away from the private jets, army of nannies and gold-plated dummies.
What are your biggest parental sporting disasters?
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The time has come for Novak
So we've already got our first Finals trend.
Andy Murray said his journey via boat to the O2 "wakes him up" - and Stan Warwinka posted a picture of himself on the Thames as he meandered towards this afternoon's crushing of Tomas Berdych.
Now Novak's joined in - snapping a picture of the Tower of London on his aptly-stencilled transport, tweeting: "On the way to the The O2 Arena. 'The time has come'. Idemo!"
He'd make a good PR man would Djokovic.
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Hello! Welcome to live text commentary of Novak Djokovic's Group A clash against Marin Cilic in the ATP World Tour Finals.
Can Novak Djokovic make it three ATP World Finals titles in a row? His bid to emulate Ivan Lendl's hat-trick of successive titles begins this evening against US Open champion Marin Cilic. They're about to get down to business.
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Since October, the world's number one tennis player has been changing nappies, shopping for buggies and singing Serbian nursery rhymes*. Recently married and a father of a baby boy - Novak Djokovic has it all. But for the next week the 27-year-old needs to prove he's still the daddy on the court.
(*I'm not entirely sure he's been singing nursery rhymes - but who doesn't?)