Novak Djokovic and Juan Martin del Potro won their Western & Southern Open quarterfinals in straight sets on Friday, setting up a reprise of their bronze medal match at the Olympics. Del Potro won that one.

He won that match," Djokovic said. "It happens. You lose, you win, but the most important thing is to try to move on and become even better and even stronger from those experiences. And try to win."

Del Potro has been nursing a sore left wrist that he'll have examined before the U.S. Open, which he won in 2009. He beat France's Jeremy Chardy 6-1, 6-3 to reach the semifinals. Djokovic knocked off Cilic 6-3 6-2, while Serena was shown the door by Kerber 6-4 6-4. The Argentine missed most of the 2010 season while recovering from surgery on his right wrist. The doctor who performed that surgery will look at the other one following the Cincinnati tournament.

"The good thing: I'm not getting worse, so I can play," he said. "But anyway, I want to wait (for) what the doctor says because I have experience on my other wrist."

Djokovic reached a Masters semifinal for the second straight week by knocking off Marin Cilic 6-3, 6-2. He won the Rogers Cup in Toronto last Sunday.

The Serb was fresh on Friday, having spent only a half-hour on court the previous day. Russia's Nikolay Davydenko had to quit after losing the first set 6-0 on Thursday night because of a sore shoulder that interfered with his serve.

"It was my best match so far here in this tournament," Djokovic said. "Came in the right moment, really." Top-ranked Roger Federer beat Mardy Fish 6-3, 7-6 (4) in an evening match to reach the semifinals. He'll play Stanislas Wawrinka, who beat Milos Raonic 2-6, 7-6 (5), 6-4 in the afternoon.

Federer skipped the Rogers Cup, taking a week off the tour after playing the Olympics. He's been sharp on the hard courts this week, playing what he called "high-quality tennis." "I played really well tonight," said Federer, who never faced a break point. "Mardy was able to lift his game in the second set."

On the women's side, Serena Williams suffered a surprise defeat in her quarterfinal while big sister Venus progressed to her first semifinal of the year. Serena Williams had her 19-match streak ended by Germany's Angelique Kerber, a 6-4, 6-4 defeat that marked the first time she'd even lost a set since her title at Wimbledon. The third-longest winning streak of her career didn't end quietly. She flung her racket at the ground, picked it up and slammed it on the court again as the second set slipped away. "I probably need a break," she said.

While Serena fumed over missed shots, older sister Venus beat Samantha Stosur 6-2, 6-7 (2), 6-4. Serena Williams came into Friday's match with a 64-4 record in the U.S. hard-court season over the past two years. Against Kerber, her game fell apart. "She's played so many matches since Wimbledon," Venus Williams said, as her sister struggled on the court. "I don't think anyone has played as many matches and played as successfully as her.

"Some rest is in order for her." For Venus, winning a tough three-setter against former U.S. Open champion Stosur was an indication she's learning to manage an immune system disorder that leaves her tired. "For me and everything that's gone on in my life, it's a huge achievement and I want to take it further," she said.

Rain interfered with two singles matches Thursday night, forcing tournament organizers to juggle the schedule.

Sixth-seeded Caroline Wozniacki was down a set to Russia's Anastasia Pavlyuchenkova when the rains came. Pavlyuchenkova finished off a 6-4, 6-4 win Friday morning, then had eight hours to rest before playing a quarterfinal match against fourth-seeded Petra Kvitova.

No. 9 Li Na's match against Sweden's Johanna Larsson was postponed before they could take the court Thursday night. Li needed only 64 minutes to win 6-2, 6-2 on Friday morning, leaving her less than seven hours to get ready for her quarterfinal match against top-seeded Agnieszka Radwanska of Poland.