LONDON — Rafael Nadal defeated Novak Djokovic 7-5, 6-2 at the ATP World Tour Finals on Wednesday, his Serbian opponent bothered by a problem with his contact lenses that forced him to leave the court.

LONDON — Rafael Nadal defeated Novak Djokovic 7-5, 6-2 at the ATP World Tour Finals on Wednesday, his Serbian opponent bothered by a problem with his contact lenses that forced him to leave the court.

There were visual issues as well in the earlier Group A match. Andy Roddick was angered by flashing lights behind the baseline during his racket-busting 7-5, 6-3 loss to Tomas Berdych that left him with an 0-2 record at this season-ending tournament.

Djokovic, who lost to the top-ranked Nadal in this year's U.S. Open final, called for a timeout out early in the eighth game. After Nadal held at love to make it 4-4, Djokovic left the court to tend to the problem, but it seemed to persist when he returned.

Nadal, who lost all three of his matches at O2 Arena last year, raised his record to 2-0 at the round-robin event for the world's top eight players. He next faces Berdych, whom he beat in this year's Wimbledon final.

All four players from the group are still in contention for the semifinals. The same is true in Group B, which features Roger Federer, Andy Murray, Robin Soderling and David Ferrer.

A few games after Djokovic called for the break, the third-ranked Serb summoned a trainer. Djokovic sat in his changeover chair as medical personnel examined his eyes . He slammed a bottle of water on the court, then slapped his hand twice into the armrest.

Nadal, who lost all three of his matches at the O2 last year, was in command this time. The Spaniard broke to open the second set and again in the third game to lead 4-0. He finished with 15 unforced errors to Djokovic's 31.

Roddick's outburst came in the second set. The neon advertising lights on the front of the boxes where the line judges stand began flickering, red and then back to their usual light blue.

After he was broken for the second time — putting Berdych up 3-2 — Roddick snapped. Or, actually, the rim of his racket snapped.

"I was angry with myself and there wasn't anybody else to talk to at that moment," Roddick said.

Once Berdych went ahead, Roddick whacked a ball high into the seats. Then, when he was walking back to his chair for the changeover, he smashed his racket against his foot and drew a code violation from the umpire.

"They were still advertising fun stuff," Roddick said. "When you're trying to track a ball, it's kind of neon lights and stuff. Then Tomas noticed it. A couple of them just went out before we played a point."

The match finished with the lights off.

Berdych, a Czech ranked No. 6, beat the No. 8-ranked Roddick for the first time in four matches this year. Berdych is making his debut at the ATP finals and is 1-1 at the round-robin event for the world's top eight players.

"There's still a chance," Berdych said. " Now it's down to myself and my next opponent."

And he knows how difficult that cane be.

"Playing Rafa, it's always great experience," Berdych said. "Even if you play on clay, grass, indoor, outdoor, whatever."

In doubles, Daniel Nestor of Canada and Nenad Zimonjic of Serbia qualified for the semifinals by beating Mahesh Bhupathi of India and Max Mirnyi of Belarus 7-6 (5), 7-6 (1). Wesley Moodie of South Africa and Dick Norman of Belgium topped Lukasz Kubot of Poland and Oliver Marach of Austria 6-1, 6-3.