Milos Raonic lost for the second straight week in an ATP quarter-final, falling to Stanislas Wawrinka on Friday at the Western & Southern Open in Cincinnati, Ohio.

Raonic, from Thornhill, Ont., now heads to the U.S. Open after losing his first meeting with Wawrinka 2-6, 7-6 (7-5), 6-4.

The 21-year-old came to the court leading the tournament with 54 aces and added 17 in his losing effort over nearly two and a half hours. Raonic ended with 36 winners and 38 unforced errors, converting on only two of his six break chances.

"I would have done a lot different," said the disappointed Raonic. I started well but I was waiting for him to lose the match instead of me winning it.

"It hurts to lose like that, it went by fast at the end. I was too passive in the second set. I needed to create more chances for myself."

Raonic dominated in the 33-minute first set as Wawrinka struggled to find his rhythm. But once the Swiss player got comfortable his heavy shots began to fall against Raonic, who was playing Cincinnati for the first time.

Raonic played level throughout the second set, which went into a tiebreaks. Wawrinka then levelled the sets at one apiece after more than an hour.

In the third set, Wawrinka went up after an early break and finished off the victory on a second match point after missing an overhead on his first.

Raonic, who is ranked 19th and will be seeded when the U.S. Open begins a week from Monday, also lost in the quarters last week in Toronto at the Rogers Cup to American John Isner in straight sets.

Raonic stands 33-14 on the season with titles at Chennai and San Jose in the first two months of the year.

He remains optimistic heading to New York.

"I'm on the right track, I'm playing good tennis, I have some things to change but they are not that big.

"I feel like I can do something at the Open. I feel I'm well-prepared and more experienced in what I have to do to win."

Djokovic, del Potro set for showdown

Novak Djokovic and Juan Martin del Potro won their Western & Southern Open quarter-finals in straight sets, 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.

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 Wawrinka.

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 quarter-final 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.

Serena fuming

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.

Her semifinal opponent will be China's Li Na, who breezed through two matches; one left hanging by the rain that curtailed play late Thursday.

After completing a 6-2, 6-2 win over Johanna Larsson, she cruised past top-seeded Agnieszka Radwanska 6-1, 6-1.

Kerber's semifinal opponent will be fourth-seeded Petra Kvitova. The Czech beat Russia's Anastasia Pavlyuchenkova 6-3, 7-6 (4).

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. .

With files from The Associated Press