World number one Novak Djokovic put his recent injury troubles behind him to grind out a three-set victory over Tomas Berdych at the ATP World Tour Finals.
Abdominal, back and shoulder problems have limited Djokovic to only seven matches since winning his third Grand Slam title of the year at the US Open.
But the Serbian proved his fitness and desire by coming from behind and saving a match point to win 3-6 6-3 7-6 (7-3) in the night match at London's O2 Arena.
The result means Wednesday's Group A matches will see Djokovic play David Ferrer and Berdych face Andy Murray - if the Briton recovers from a groin complaint in time.
Djokovic has enjoyed one of the greatest seasons in tennis history, recording a 41-match winning streak from January to June and collecting three Grand Slam titles and the number one ranking.
He will be delighted to get his season-ending championship bid off to a successful start - and extend his 2011 win-loss record to 70-4 - following a string of physical issues since New York.
Djokovic insisted before the match that he was "100% ready" for one last push, but it did not seem that way as Berdych powered into a 4-0 lead at the start of the first set.
The 6ft 5in Czech had won just one of their previous eight meetings but arrived high on confidence following his victory over Murray in the Paris Masters quarter-finals.
He quickly set about exploiting an out-of-sorts Djokovic with a succession of booming serves, monstrous groundstrokes and irretrievable volleys at the net.
A lapse in concentration from Berdych - including two horribly wayward forehands - let Djokovic secure his first game and bring up three more break-back points in game seven.
But the seventh seed saved them all and completed a superb set with a fine backhand up the line, his 10th winner to Djokovic's three. The world number one posted 15 unforced errors.
However, it was a case of roles reversed at the start of the second, Berdych's forehand malfunctioning to help Djokovic go 3-0 ahead.
The crowd were then treated to a number of breathtaking exchanges, both players mixing attack with defence to draw several standing ovations.
Yet neither man particularly troubled the other and consecutive aces gave Djokovic a love hold in game nine to level the match.
The tension was palpable as the deciding set went with serve until game six, when two unstoppable forehands brought Berdych a break to 15 and put victory within his grasp.
That only seemed to prompt nerves, though, and Djokovic responded immediately.
Berdych squandered a glorious opportunity to post a famous triumph when he held to love for 6-5 and was then gifted a match point via a Djokovic double-fault.
But he unleashed two woeful forehands to blow his chance, and a couple more basic errors in the tie-break were enough to confirm the win for Djokovic.
The world number one admitted he had been fortunate to win the match.
"I wasn't very satisfied with my performance," he said. "I know I wasn't playing on top of my game.
"But a win is a win in the end. I knew my opponent is an unpredictable player who has powerful groundstrokes and a great serve. If he's on, if he feels the ball well, it's difficult to compete with him. That's what happened.
"I was too passive. But it was important to stay in the match and use the chances when they were presented."
Djokovic insisted the shoulder injury he had coming into the tournament had not troubled him during the match.
"That's something I'm happy about, my condition," he said. "I haven't felt any pain in my shoulder."
Berdych insisted he had no regrets about his tactics on match point. "Probably I would try to do the same if I would have a match point again," he said. "Maybe I was too risky? But that's how I play. That was my chance to end the match."
In the doubles competition, top seeds Bob and Mike Bryan had to save a match point before beating seventh seeds Jurgen Melzer of Austria and Philipp Petzschner of Germany. The Americans lost the first set on a tie-break (4-7), levelled with a 7-5 win in the second set, and then won the match tie-break 10-7.
Also in Group A, sixth seeds Robert Lindstedt of Sweden and Horia Tecau of Romania beat fourth seeds Mahesh Bhupathi and Leander Paes of India 7-6 6-1.