Djokovic beat Davydenko to win the final of this prestigious end-of-year event 12 months ago and the world number three, who also defeated the Russian in the group stage last year, came from behind to frustrate him again in his opening Group B match at London's O2 Arena.

Robin Soderling's surprise victory over Rafael Nadal earlier on Monday had thrown the group wide open and Djokovic looks in the mood to mount a strong bid to retain the title.

Djokovic said: "He was a better player in the first two sets and maybe overall in the whole match. But when you have bad days, you still find a way to win. I think that's something that is great to have in your game."

After reaching the final last year, Davydenko is in danger of being eliminated at the group stages this time and will have to beat Nadal today to keep his hopes alive.

Nadal is still some way short of his best after a traumatic, injury-hit season and Davydenko believes the Spaniard is struggling on the fast indoor court.

"There is a difference between clay court and indoor. Sometimes it is pretty fast for him indoors," Davydenko said. "That's why he was losing control.

"He cannot always control from baseline. That was very difficult for him.

"I know I showed my good tennis and I still have a chance in the tournament.

"We'll see because in the next match I have against Nadal. Maybe I have some confidence for that."

Davydenko's own chances of victory could be hampered by breathing difficulties that have bugged him for several weeks.

He called for a doctor in the second set because of the problem and added: I have a problem with my breathing. I already see doctors in three different countries. It's really a problem.

"Some doctors say maybe asthma, some say maybe allergy, some say bronchitis. I really don't know what it is."

Davydenko's metronomic accuracy kept Djokovic from taking control early on and the world number six got the first break in the fifth game.

There was a brief glimmer of hope for Djokovic in the next game as he earned two break points but he squandered both opportunities.

That proved the Serb's last chance of the set as Davydenko broke again in the ninth game to move in front.

After a tight start to the second set, Djokovic got the stroke of luck he needed to get back into the match.

At 4-4 and 30-0 down on Davydenko's serve, a Djokovic return clipped the net and fell kindly on the other side. Djokovic crossed himself in thanks for the good fortune and proceeded to reel off three more points to break for the first time before serving out the set.

Both men were playing with an intensity far greater than anything shown by the likes of Nadal and Roger Federer in their opening matches. There was little to separate them and the final set followed the same pattern. Djokovic broke Davydenko in the first game and saved three break points in a marathon sixth game.

Then Davydenko made a final push and broke as Djokovic served for the match at 5-4. But Djokovic broke back to love and held his serve this time to clinch the win in two hours and 46 minutes.