1653: Right, that is the afternoon session of action done and dusted at the O2 Arena. Swing back over here shortly before 1945 GMT and we'll see how David Ferrer and Janko Tipsarevic shakes down and what the semi-finals look like at the end of play.
1645: The tale of the tape is quite revealing from Juan Martin del Potro's win. Roger Federer won a decent 78% and 61% of his first- and second-serve points respectively, but Del Potro's serve stats are tidier stuff. Eighty-one percent of first serve points won by the Argentine with 74% hoovered up behind his second. He converted his one and only break point in the match as well.
Stephanie Siu on Twitter:"So Federer beats Ferrer, Ferrer beats Del Potro, Del Potro beats Federer. I feel a new version of Rock Paper Scissors coming on."
1637: There is British representation in the doubles as well as in the singles at the World Tour Finals this year. Just as at Wimbledon this year, Jonathan Marray has gone one better than Andy Murray, qualifying at the top of his group. He and Freddie Nielsen will face either world number one pairing Bob and Mike Bryan or Marcel Granollers and Marc Lopez in the semi-finals depending on the latter's result this evening.
1633: A consistently fit and firing Juan Martin del Potro could spice up the very top of the men's game particularly in Rafael Nadal's absence. As unfortunate as it is for David Ferrer not to make it through, is Del Potro, with age on his side, the likely to close that gap on the so-called "Big Four"?
Juan Martin del Potro: "I'm so happy to beat Roger once again, I know he's a favourite here but the crowd respected me also.
"It's going to be a tough match against Djokovic or Murray, I don't know yet but I'm so excited to get into the semis.
"I think I was a little lucky towards the end of the match. Always when you beat these kinds of players you need luck on your side but I played well. I took the breakpoint chances in the last set then I served really well in the last game. I was very nervous but I closed out the match."
John Thompson on Twitter: "If DP wins in three, Ferrer can't qualify but CAN influence the semi line-up. Bizarre."
1625: That result renders tonight's match between David Ferrer and Janko Tipsarevic a dead rubber in terms of qualification. Neither can make it through to the last four, but they will decide who plays whom in the semi-finals. A victory for Tipsarevic would mean Federer v Djokovic and Murray v Del Potro. A win for Ferrer would make it Federer v Murray and Djokovic v Del Potro.
JUAN MARTIN DEL POTRO BEATS ROGER FEDERER 7-6 (7-3) 4-6 6-3 TO QUALIFY FOR THE WORLD TOUR FINALS LAST FOUR
Federer 6-7 (3-7) 6-4 3-6 Del Potro
Two match points for Del Potro as he muscles his way to 40-15. Federer dawdles at the back of the court. A spot of gamesmanship? It doesn't save the Swiss. Great retrieving from Del Potro and Federer cannot find the killer shot as he comes to the net. His stretching attempt at a backhand volley is planted into the net and Del Potro has back-to-back wins against Federer.
Federer 6-7 (3-7) 6-4 3-5 Del Potro*
A Roger Federer double fault gives Del Potro 15-15, but the Swiss spots his opponent slipping a little deep behind the baseline on the next point. A tickled drop-shot is too good for Del Potro, whose bodyline bullet in the next rally produces a sharp volleyed response from Federer. Federer wraps up the game with an ace but need a break to avoid defeat in the next.
Travis on Twitter: "If Federer is leaking unforced errors like this then surely it's Murray's best chance to get a win over him indoors in the semis?"
*Federer 6-7 (3-7) 6-4 2-5 Del Potro
It has taken 23 minutes of the third set for Federer to win a point on the Del Potro serve. The Swiss moves 0-15 up but Del Potro is not going to give him a second in a hurry. He rattles off four unanswered points of his own to move within a game of victory.
Federer 6-7 (3-7) 6-4 2-4 Del Potro*
A horribly shanked backhand, more graphite than cat gut on the shot, gives Del Potro a sniff of a chance at 30-30. There is a little gasp from the crowd as well at the sight of that beautiful action delivering such an ugly result. Federer lands two timely aces to put himself out of harm's way, but he is looking anemic out there.
*Federer 6-7 (3-7) 6-4 1-4 Del Potro
Roger Federer has the whip hand as he advances to the net at 30-0 adrift, but a limp backhand volley at the net lacks the direction and touch to keep it out of Juan Martin del Potro's clutches. The Argentine bounds forward from the baseline and strokes the ball back past a rooted Federer. A big serve sees out the game and Federer's game is in need of the jump cables to get back into the contest.
David Law on BBC Radio 5 live sports extra's online commentary: "Federer made a little noise of disgust in that last game, and he must have known that something is off with him at the moment, and he is very concerned."
Federer 6-7 (3-7) 6-4 1-3 Del Potro*
Roger Federer gets his names on the board, but Juan Martin del Potro has sensed blood and is sniffing around the Swiss's ailing serve.
*Federer 6-7 (3-7) 6-4 0-3 Del Potro
Juan Martin del Potro holds serve with a commanding game and has leaned on the gradient increase buttton for Federer. The Swiss needs a hold next to avoid being spat out the back of this set before it has really got going.
Federer 6-7 (3-7) 6-4 0-2 Del Potro*
Roger Federer is under assault on the forehand side and Del Potro's barrage draw a mis-hit to bring up 30-30. The Argentine picks off Federer as he attempts to mix it up with a raid to the net on the next rally to give himself a break point at 30-40. Federer changes tack again, rallying on the backhand diagonal but Del Potro is able to match all the variation the Federer can put on the ball. Federer slumps into the net to bring the exchange to an end and concede the break.
Russell Fuller on BBC Radio 5 live sports extra's online commentary: "The possibility of Federer v Murray in the semi-finals gets closer by the moment."
*Federer 6-7 (3-7) 6-4 0-1 Del Potro
Juan Martin del Potro makes sure that there is no repeat of the lax opening service game that cost him at the start of the second set. He holds to love in wham-bam style. The Argentine can obviously no longer win in straight sets, meaning only a Janko Tipsarevic victory over David Ferrer tonight combined with a Del Potro win in this match can divert Federer away from Andy Murray in the semi-finals.
Federer 6-7 (3-7) 6-4 Del Potro*
Federer looks to have hit a winner out wide at 15-0 up but those long limbs of Del Potro extend to swipe back a vicious forehand at an even more acute angle. The Swiss gives Del Potro a real working over at 30-15, but buries what should have been the killer final shot into the net. Federer finally shakes off his pursuer though as he finds the mark with a whippy forehand and a clonking ace out wide. One set apiece.
*Federer 6-7 (3-7) 5-4 Del Potro
Roger Federer has a narrow corridor or space to aim at as Juan Martin del Potro comes to the net at 30-15. Plenty of time to line up the backhand, but the Swiss's attempted down-the-line winner is snared by the tape. Del Potro' unfurls that big serve again on the next point to take the game, but he coul have been in a bit of trouble had Federer made the backhand on the previous point. Federer to serve for the set next.
Federer 6-7 (3-7) 5-3 Del Potro*
Federer breezes through another service game and the early lapse in concentration from Del Potro in the opening game of this set may be fatal.
*Federer 6-7 (3-7) 4-3 Del Potro
Del Potro looks ready to repeat the love-hold trick as he coasts into a 40-0 lead. A brilliant bit of improvisation from Federer as he fends a ball away from his face with a frying pan-style swat checks Del Potro's progress momentarily. "Exhibition stuff," observes Andrew Castle in the BBC commentary box. Del Potro shoulders arms to a deep Federer groundstroke though and that is shrewdly done as the ball the flies just long.
Federer 6-7 (3-7) 4-2 Del Potro*
Federer's serve is flowing at a rapid rate and it washes away Del Potro with another hold to love.
*Federer 6-7 (3-7) 3-2 Del Potro
A sublime dipping, topsinning forehand from Roger Federer makes up a yard or so in flight to curl in and onto the sideline. The crowd coo appreciatively and Del Potro looks bemused, but he recovers from the shock to shut out Federer. An ominous sign of the wizard's powers returning perhaps though...
Federer 6-7 (3-7) 3-1 Del Potro*
Federer's game is looking like it has rediscovered its bite as he sharply shoots down Del Potro. In the first set he looked like he was content to wait for the Argentine to fold, which Del Potro stubbornly refused to do.
Tim Henman on BBC Two: "Federer does not want to avoid anybody on a tennis court, but he has to find something here. Del Potro has done a good job of soaking up his ground strokes."
*Federer 6-7 (3-7) 2-1 Del Potro
Juan Martin del Potro gets a foothold in the set and apparently the acting world has caught up with the sporting one. Sir Ian McKellan and Richard "Victor Meldrew" Wilson have joined Ledley King in the posh seats.
Federer 6-7 (3-7) 2-0 Del Potro*
A hold to love from Roger Federer as he consolidates that early break with an authoritative hold. Federer will lose his grip on top spot in Group B and head into a semi-final against Novak Djokovic rather than Andy Murray if he loses to Del Potro and Tipsarevic beats Ferrer tonight or a straight sets win for Del Potro combined with a Ferrer win. I think that is right. Full details over here on the ATP site.
*Federer 6-7 (3-7) 1-0 Del Potro
Roger Federer seems to have had a stiff word with himself during the changeover. He is out of the blocks like a flash in the second set, catching Del Potro cold as he clambers into a 0-40 lead. He converts the first of the three break points to take an early advantage in his search for a comeback.
Federer 6-7 (3-7) Del Potro
1510: Del Potro holds serve again to bring up 6-1 and five set points. Federer knocks back the first two with solid serving. Now the balls head up to the opposite end though and he is facing down the barrel of the Del Potro serve. A first serve flashes past Federer, but the Swiss strolls forward inspects the line and call a challenge. The tennis cyborg must have microscopes for eyes, because that was a fraction out. No matter for Del Potro though who produces a kicker out wide on his second serve to take the set.
1505: After three points on serve, Federer is the first to blink in the tie-break. It is such a big blink, it could be narcolepsy. The Swiss racks up his ninth and tenth unforced errors on the forehand side as he slips to 4-1 behind. Del Potro hold his serve to move 5-1 up and firmly in control as the players head for their chairs.
*Federer 6-6 Del Potro
A hold to love for Del Potro with Federer buckling down in preparation for the shoot-out.
Federer 6-5 Del Potro*
More like the usual service from Roger Federer as he takes the game away from Juan Martin del Potro with seven strokes of his raquet. Tie-break looming on the horizon.
*Federer 5-5 Del Potro
Straight back at Federer from Del Potro as the Argentine disposes his service game with the effeciency of a late-night trip to the recycling bins.
Andrew Castle, BBC commentator - "Juan Martin Del Potro certainly has the tools to get in among that great four-ball of Djokovic, Federer, Nadal and Murray"
Federer 5-4 Del Potro*
A cheeky 'tweener from Juan Martin del Potro as he retrieves a Federer lob doesn't win the point but certainly wins him some more fans amongst the O2 crowd. The Argentine is building up a head of steam and hammers away a volley at the net en route to deuce. Federer fluffs a regulation backhand from the back of the court to allow advantage to slip away from him, but he is eventually across the line. He is still some way short of his imperious best though and Del Potro is at least partially to blame for that.
1448: Let us know what you think of this afternoon's action, and all the various permutations still possible, by sending your thoughts to us at #bbctennis or 81111 on UK mobiles.
*Federer 4-4 Del Potro
If you are on your way to the bookies, hold those horses. Federer has Del Potro scampering back and forth on the baseline and, taking his time to twirl the raquet in his hand while he waits for the ball to drop, the Swiss hammers away a smash to move 15-40 up. Two break points, but both are back in the bag as Del Potro unleashes that big dog of a serve and then Federer is deceived by the depth of Del Potro forehand that lands right on the baseline. Federer has another break point as Del Potro missed wide with a forehand, but his groundstrokes, under pressure for Del Potro, falter again. He is not as silky smooth as he has been earlier this week and Del Potro finally nails the game via a successful challenge to a Lars Graff overrule. As you were then, get back down the bookmakers.
Federer 4-3 Del Potro*
Federer's serve is as secure as one of the banks in his native Switzerland. Del Potro takes a point of it, but there is no real danger for the six-time champion. If you had to put money on it, I'd suggest a tie-break either way would be where to put your stake on the outcome of this opening set.
*Federer 3-3 Del Potro
Del Potro is making use of every inch of that gangly frame, winding up the sinews and tendons to catapult zinging groundstrokes back into the Federer hitting zone. He holds to love with Federer pressed back and struggling to get the ball back over the net.
Federer 3-2 Del Potro*
Roger Federer slaps Del Potro round the chops with a love hold of his own. Both players' serves are still looking impregnable so far. We are still to see a serious challenge to either player on serve.
*Federer 2-2 Del Potro
Del Potro bounces back out and smacks down a series of big serves to keep pace.
Federer 2-1 Del Potro*
Juan Martin del Potro twangs the top string again on the final point of the game but this time the net cord carries his shot out of play. Federer steamrolling through the final service game before the changeover in and both player look to have hit their straps. A sighting of former Tottenham defender Ledley King in the courtside seats for the football fans amongst us.
*Federer 1-1 Del Potro
Juan Martin del Potro get the benefit of a hefty chunk of net cord that sends the ball floating past a wrong-footed Federer, taking some of the pace of it as it touches down just the right side of the baseline for the Argentine. Federer challenges the call to bring umpire Lars Graff into the match for the first time. No dice for the Swiss. 30-15 and Del Potro has enough biff in his groundstrokes to outmuscle Federer from the baseline.
Lydia Symonds on Twitter: "Federer V Del Potro is going to be a SCREAMER"
Mark Thomas on Twitter: "If you don't like watching Federer play tennis, surely it can only be because you don't actually like the game?"
Federer 1-0 Del Potro*
Roger Federer works away an angled forehand for 30-15 before a kick serve out wide stretchs Del Potro out of shape. A missed smash as he backtracks under a lofted Del Potro moon ball brings up 40-30, but Federer shuffles him out of the door as he comes to the net under the cover of canny build-up play.
*-denotes NEXT to serve
1414: As well as cracking down serves from step-ladder height, the Argentine Del Potro's flat, slugging groundstrokes that won't give Federer much to work with. Federer has the fuzz in his hands and will serve first.
1410: The players are out on court knocking up, David Federer was sent on his way to the net with a cheery thumbs-up from the tiny child mascot who led him out through the dry ice. When Federer gets to the gauze, he is greeted by the considerably more precipitious sight of the six foot six inch Juan Martin del Potro.
1407: There is, of course, a British angle on this contest as well with a Federer win setting him on collision course with Andy Murray in a repeat of this year's Wimbledon final. If Federer loses, he may still play Murray in the last four, but it will be dependent on the outcome of this evening's match between David Ferrer and Janko Tipsarevic.
1402: Juan Martin del Potro also claimed the scalp of Federer in the final of the 2009 US Open, ending a five-year unbeaten run for the Swiss at Flushing Meadows. In short, the 24-year-old has made a bit of a habit of raining on Federer's parade.
1358: But that is only the start of it. They have played another six times this year. Federer has won all but one of those meetings. The one that Del Potro won will have stung the glacial-cool Federer though. The Argentine beat Federer in his hometown to clinch the Swiss Indoors in October.
1353: The pair went traded blows all the way down the home straight of their London 2012 semi-final, with Federer finally prevailing 19-17 in the final set of a record four hour and 26 minute match.
1345: There has been a lot of talk about the burgeoning rivalry between Novak Djokovic and Andy Murray this week. This afternoon is the latest installment in what has been another tasty little duel all season long; Juan Martin del Potro against Roger Federer.