ATP World Tour Finals
Venue: O2 Arena, London Date: 22-29 November
Tournament coverage: Live on BBC Two, 28/29 November, 1430 GMT; every match live on BBC Radio 5 live/5 live sports extra; live text commentaries on BBC Sport website; live coverage every day on Sky Sports
Djokovic has now won 11 matches in a row including back-to-back titles
By Piers Newbery
BBC Sport at the O2 Arena
Defending champion Novak Djokovic overcame a sluggish start to find a way past Nikolay Davydenko in a dramatic match at the ATP World Tour Finals in London.
In a repeat of last year's final, the Serb came through 3-6 6-4 7-5 in Monday's evening session as the two remaining players began their campaigns in the eight-man tournament.
Djokovic has been the man to beat in the build-up to London with three victories in his last four tournaments, including wins over Roger Federer and Rafael Nadal.
But the 22-year-old struggled for form and composure in his first match at the O2 Arena as he battled past seventh seed Davydenko in two hours 45 minutes to join Robin Soderling at the top of Group B, after the Swede earlier upset second seed Nadal.
"He was the better player in the first two sets," said Djokovic after his hard-fought win.
"I was lucky to get through and win this. I was fighting for every service game and in the end I'm happy with the performance."
But he added: "I haven't felt great on the court today - I'm definitely not the fittest guy on the tour right now.
"The confidence is there, the game is there, but physically you can't fight nature sometimes. You can't do much about it."
Davydenko admitted: "I lost concentration and started to have problems with my body, called the doctor but couldn't forget it in my mind for the next few games and started to lose my serve. Then it was difficult to get it back."
Djokovic had lost just one of his 19 matches since the US Open but that defeat came against Davydenko as the Russian stormed to the title in Shanghai last month, and with a 2-2 head-to-head record there was every reason to expect a tight match.
The early exchanges suggested Djokovic was still feeling the adrenaline rush of back-to-back wins in Basel and Paris as he wound up some heavy forehands, but Davydenko withstood the barrage and made the breakthrough himself.
Djokovic opened game five with a double-fault and, after a forehand winner from Davydenko, successive errors from the Serb handed over the break before the defending champion saw two break points go begging in the following game.
The Russian's ability to serve out the set was not fully tested when, at 5-3, Djokovic double-faulted on the first point of a game for the second time, soon found himself two set points down, and then watched as a Davydenko sliced return floated past him into the corner.
The calm, confident Djokovic now gave way to the more petulant side of his character and he slammed his racquet into the floor as Davydenko began the second set in confident fashion.
Djokovic's mood did not improve when he could not capitalise on three gifts from his opponent at 2-2, failing to make a return with two break points at 15-40, and there was plenty of shrugging and looking to his coaches as the set progressed.
In the end, it was Davydenko who brought the world number three back into the match with a poor game at 4-4, making a backhand error before going for too much on a cross-court forehand and dragging it into the tramlines on break point.
Davydenko then called for the doctor at the changeover but showed no sign of discomfort when he returned to the court.
Davydenko lost to Djokovic in the 2008 ATP Tour final
Djokovic was fully engaged again but the nerves had not disappeared and another double-fault saw him fall to 30-40.
It was a crunch moment as Djokovic faced throwing away his hard-won advantage, but the former Australian Open champion fired a big serve down middle and then converted his first set point with a thumping forehand.
The celebration was as much in relief as joy, and Davydenko gave him a further boost at the start of the final set with two wayward forehands as he dropped serve immediately.
Neither player was able to find their best form consistently in an edgy decider, but Djokovic clung to his lead, fighting off three break points in a tense game six, before moving within sight of victory at 5-4.
By now, Djokovic was bouncing the ball seemingly endlessly ahead of each serve - always a sign of nerves from the Serb - and Davydenko kept the match alive with a magnificent backhand winner.
But it was the last stand from the Russian who was swiftly broken to love in the next game, and at the second time of asking Djokovic finally got himself over the finish line on his second match point to bring the match to an end at 11.49pm.
By that time a good number of the capacity crowd had already headed for the tube station with the last train imminent, but those who stayed enjoyed the most dramatic match of the two days, if not the best tennis.
Djokovic now faces Soderling on Wednesday but has played down his chances of beating the Swede.
"I'm very exhausted because of the long season," said the Serb. "I wouldn't rate myself as a favourite in that match because he's in great form. It's going to be a tough one.
"I finished very late today so I hope the free day tomorrow will help me to recover."
The doubles event continues to spring surprises, with sixth seeds Lukasz Kubot and Oliver Marach beating fourth seeds Lukas Dlouhy and Leander Paes 6-4 7-6 (7-3) in the first evening match.