Roger Federer of Switzerland reacts during his men's singles tennis match against Novak Djokovic of Serbia at the ATP World Tour Finals at the O2 Arena in London Nov 5, 2013.[Photo/Agencies]

'Toughest start'

With the majority of the 17,000 crowd, many dressed in Swiss red and wearing Federer-branded clothing, roaring on the 32-year-old he battled back from the disappointment of dropping serve in the 10th game to lose the first, levelling the match with a sublime tiebreak which he won 7-2.

After spluttering into life, however, Federer lost his spark in the deciding set as Djokovic repeated his victory over the Swiss in Paris at the weekend.

"It's slow here and it's the first match, so there's going to be errors," Federer, who had not lost his opening match at the Tour Finals since 2008, told reporters.

"There's a lot of neutralizing going on. You have to take a lot of chances to get the ball past Novak, like I mentioned. Eventually that draws errors out of you."

Defending champion Djokovic, who was in action in Paris until Sunday and had precious little time to adapt to the conditions in London, looked weary at the end of the second set but struck back immediately at the start of the decider.

Federer's resistance melted away after he had been broken to love in the first game of the decider and there was no way back when he surrendered serve again to trail 4-1.

Federer, who has won a record six titles at the year-ender, remained hopeful of beating Richard Gasquet on Thursday to keep alive his hopes of ending a disappointing year on a high.

Djokovic will face Juan Martin del Potro on Thursday and said he was relieved to survive a tough opener.

"It was probably the toughest start I could get for this tournament, especially considering the scheduling that I had, playing two days ago in Paris finals," he said.

"I didn't know how I would respond physically. It took some adjustment, but I'm happy with the way I played. I'm just happy that I overcome this challenge."