Roger Federer had never lost his opening match at the ATP World Tour Finals since the elite tournament moved to London in 2009.

That's all changed now.

On the back of a chaotic and disappointing season, the former world No 1 tasted defeat for the second time in less than a week against defending champion Novak Djokovic, 6-4, 6-7 (2), 6-2, on Tuesday.

The Serb extended his winning streak to 18 matches after capturing his sixth title of the season last week in Paris, where he defeated Federer in the semifinals, and kept alive his chances of overtaking Rafael Nadal in the race for the year-end No 1 spot.

"It was a big challenge physically," Djokovic said. "Only 48 hours ago I was playing in Paris, so to be able to play this well and beat Roger is incredible.

"First of all, I have to enjoy the win and then think about the recovery because it's a big win. The good thing is, there is a day off. I'm going to try to recharge my batteries, mentally and physically."

This 31st encounter between Federer and Djokovic was somewhat disjointed, with unforced errors and flashes of brilliance leaving both looking exhausted.

"I actually felt physically fine," said Federer. "So I regret not having taken better advantage of my chances."

The oldest man in the draw at 32, Federer refused to draw early conclusions from a season that saw him suffer from back problems and drop to seventh in the rankings.

"The season is not done yet," he said. "Let's talk about this stuff in hopefully four matches."

Meanwhile, Djokovic said he has lost all trust in the anti-doping program in tennis and fears he could become a victim of its "negligence" after fellow Serb Viktor Troicki failed to have his doping ban overturned on Tuesday.

Troicki, who was banned after failing to provide a blood sample in April, had his sanction reduced to 12 months by the Court of Arbitration for Sport, ruling him out until next July.

"First of all, Viktor is a very good friend of mine," said Djokovic. "It's very bad news that we got for him, and for me, for all of us who are close to him. But I think it's just not bad news for him, it proves again that this system of WADA .... does not work.

"I don't have trust in them anymore. I don't have trust in what's going on.

"I don't know if tomorrow the representative, the DCOs (doping control officers) who are representatives of WADA are there at the tournament, because of their unprofessionalism, because of their negligence, because of their inability to explain the rules in a proper way. I don't know if they're going to misplace the test that I have or anything worse than that."