LONDON (AP) -- Defending champion Novak Djokovic joined top-ranked Rafael Nadal in ATP World Tour Finals semifinals, overcoming Juan Martin del Potro on Thursday.
Djokovic, undefeated in last year's Finals, improved to 2-0 in Group B with a 6-3, 3-6, 6-3 victory over the hard-hitting Argentine.
"With his serve it's definitely a big advantage if I can get a break advantage and close it out," said Djokovic, who saved two break points early in the decider before taking del Potro's serve for a 4-2 lead. "I'm very glad I could win."
Djokovic beat Roger Federer in his opening match.
Federer made light work of Richard Gasquet for his first round-robin win, beating the Frenchman 6-4, 6-3 to boost his chances of making the semifinals for an 11th time.
Nadal qualified from Group A on Wednesday when he beat Stanislas Wawrinka to clinch the season-ending No. 1 ranking for the third time in his career.
Djokovic and del Potro couldn't reproduce their epic Wimbledon semifinal. The in-form players of the autumn exchanged groundstrokes early on, with Djokovic coming out on top of long rallies.
Playing more accurately and trying to shorten the points, del Potro soldiered on in the second set and was rewarded for his patience as he converted his first break point of the set in the sixth game thanks to a lucky net cord. The former U.S. Open winner then held his next two service games to force a third set.
In the third game, del Potro had 15-40 after Djokovic's first double fault of the match. The Serb saved the first break point with an ace and del Potro wasted the other with a botched forehand. The episode sapped del Potro's morale and he lost his serve at love in the sixth game. It was decisive.
In a match pitting together two stylists, Federer and Gasquet were ordinary. Federer quickly took control and broke Gasquet four times in the match. The Frenchman hit only 11 winners and looked inhibited throughout.
Federer, who has won a record six year-end championships, was not having a great day either - he had 30 unforced errors - but prevailed on important points as Gasquet converted only one of his six break chances. Federer needed five match points to earn his 43rd Finals win.
"It is a big win for me and hopefully I can keep it up in the next match against Juan Martin del Potro," said Federer, chasing only his second title of the year.
"I'm coming back from a lot of sort of ups and downs, resetting things, trying out things, making sure I get my confidence back, my movement and so forth."
Only once in his previous 11 Finals has Federer failed to advance to the last four. A win against del Potro on Saturday will guarantee him a spot in the semis, while Gasquet became the second player to be eliminated, following David Ferrer.
It was Federer's 11th win in 13 matches against Gasquet, who started by winning his first service game at love before fading.
The Finals are Federer's last chance to win a big tournament this season. He has won only one title in the past 14 months and has only three victories against top-10 opponents in 2013, including one against del Potro in Paris last week.
"I'm glad I got one back in Paris because I knew we could be again in the same group, we could be playing each other," Federer said. "I think that win for me in Paris psychologically was very important. I could beat top-10 guys, I beat Richard, I beat Juan Martin last week. I truly believe my confidence is higher, and that's what I need to beat the best, and Juan Martin is part of that group honestly."
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