LONDON (AP) -- 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, improving 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 winning streak to 18 matches after capturing his sixth title of the season last week in Paris. He beat Federer in Paris in the semifinals.
Djokovic is trying to finish No. 1 for the third year in a row.
Top-ranked Nadal made a winning return at the ATP World Tour Finals, cruising past fellow Spaniard David Ferrer 6-3, 6-2 Tuesday in his first round-robin match.
Playing for the first time in two years at the season-ending tournament, Nadal traded breaks early in the match. Then his serve improved and he dictated play from the baseline and forced the third-ranked Ferrer into numerous errors. Nadal converted six of seven break points.
Nadal is bidding to win the elite event for the first time. He'll maintain the year-end No. 1 spot if he wins a second match at O2 Arena.
He claimed his 21st win in 26 matches against Ferrer - but only his first indoors - and avenged his loss in their Paris Masters semifinal last week.
"He played a lot better than me in Paris, but today I played a little bit better than him and it's important for my confidence to start with a victory after Paris," Nadal said "There is no rest, but the energy of this stadium is fantastic."
Nadal and Ferrer play in Group A, alongside Stanislas Wawrinka, who beat Tomas Berdych in his opening match on Monday. No. 2 Novak Djokovic and Roger Federer, playing later Tuesday, are in Group B with Juan Martin del Potro and Richard Gasquet.
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