SHANGHAI - Defending champion Novak Djokovic shrugged off a injury scare to win his Shanghai Masters opener on Wednesday, keeping alive the prospect of a mouthwatering quarter-final against Roger Federer.

In the late-evening match in second-round play, newly re-crowned world number one Rafael Nadal polished off Ukraine's Alexandr Dolgopolov in less than an hour, with just five unforced errors in a 6-3, 6-2 win. Top seed Djokovic cantered through the first set against Spain's Marcel Granollers for the loss of only two games. But early in the second he pulled up in obvious discomfort and called for the ATP trainer, who applied heavy taping to his right foot and ankle. Despite the medical treatment, the 26-year-old did not look inconvenienced as he returned to the court to seal a 6-2, 6-0 victory, hitting a total of 26 winners.

"It was just an awkward movement. I was in an off-balance position. I was running and I made a quick movement that caused a sharp pain at that instant moment," said Djokovic. "But after the match we determined that it's nothing serious, so it's all fine." Federer, who has slipped to seventh in the world rankings and faces a battle to qualify for next month's World Tour Finals in London, won 6-4, 6-3 against Italian Andreas Seppi after a testing start.

The Swiss great, playing his first match since a disappointing US Open, lost his serve early but broke back immediately against the world number 22, repeating the feat to take the opening set. A single break in the second was enough to seal the match. Federer, who had a mixed bag of 19 winners and 29 unforced errors, said going a set up and then a break up in the second set was key for him and he started to feel comfortable. "Even though it remained tough, Seppi did a good job of hanging around. That's the kind of match you want in this particular situation I'm in right now."

Earlier, German world number 23 Philipp Kohlschreiber rattled a feverish Juan Martin del Potro, striking 50 winners and 22 aces in a brave display of attacking tennis that had the Argentine searching for answers before he dug deep to win 3-6, 6-3, 7-6 (7/4). "Last night I couldn't sleep really well," said the sixth seed, fresh from winning the Japan Open. "This morning I took ibuprofen, a couple of pills, trying to help me feeling better. But I'm not 100 percent yet."

"I think my serve helped me a lot to keep playing for the three sets because I made many aces," added Del Potro, who hit 14 aces.  Third seed David Ferrer beat Lukas Rosol 6-3, 6-4, fourth seed Tomas Berdych beat Feliciano Lopez 7-6 (7/5), 6-2 and seventh seed Jo-Wilfried Tsonga eased past Pablo Andujar 6-3, 6-2. There were also wins for seeds Tommy Haas, Stanislas Wawrinka and Kei Nishikori. But America's top player John Isner lost in straight sets to Argentina's Carlos Berlocq.

Spain's Tommy Robredo retired from his match against Italy's Fabio Fognini. Djokovic, Nadal and Ferrer have already qualified for the end-of-season showpiece in London. Britain's Andy Murray has withdrawn from the tournament as he recovers from back surgery. Drenching rains earlier in the week as a result of Typhoon Fitow forced matches indoors in Shanghai, but the wet weather relented on Wednesday.