Defending champion Novak Djokovic defeated six-time winner Roger Federer 6-4, 6-7 (2), 6-2 at the ATP World Tour Finals on Tuesday, keeping alive his chances of overtaking Rafael Nadal in the race for the year-end No. 1 spot.

Playing his first round robin match at the elite tournament this year, Djokovic extended his current winning streak to 18 matches after capturing his sixth title of the season last week in Paris, where he defeated Federer in the semifinals.

Djokovic is trying to finish No. 1 for the third year in a row but Nadal, who eased past David Ferrer 6-3, 6-2 earlier Tuesday, needs only one more win in London to be guaranteed of the top spot.