Del Potro, Djokovic reach Cincinnati semis

Novak Djokovic, from Serbia, hits a forehand shot against Marin Cilic, from Croatia, during a quarterfinals match at the Western & Southern Open tennis tournament, Friday, Aug. 17, 2012, in Mason, Ohio. Djokovic won 6-3, 6-2. (AP Photo/Al Behrman)

MASON, Ohio— Only one of the Williams sisters is heading to a semifinal, and it’s not the one with the long winning streak. Serena Williams lost that — and her cool — at the Western & Southern Open on Friday.

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 had another jump-for-joy day. She reached the semifinals of a tournament for the first time this season by beating Samantha Stosur 6-2, 6-7 (2), 6-4.

On the men’s side, Novak Djokovic and Juan Martin del Potro won in straight sets, setting up a rematch of their bronze medal match at the Olympics. Del Potro won that one.

After all of her summer success on Wimbledon’s grass, Serena Williams is having a tough time getting readjusted to hard surfaces. One of the best stretches of her career included single and doubles gold medals at the Olympics in London.

She came into Friday’s match with a 64-4 record in the summer hard-court season over the last two years. Against Kerber, her game fell apart.

She missed so many easy shots that after she hit an overhead that landed in the court, she smiled and raised her arms in mock triumph.

Her sister thinks the last few weeks have caught up with her.

“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.

“I think some rest is in order for her.”

Wearing an Olympics shirt after her match, Serena Williams agreed she needs a little time to get recharged for the U.S. Open.

“I’ll be better in New York, I promise,” she said.

First, she’s going to stick around to watch her sister’s resurgence. Venus Williams raised both arms and jumped — kicking up her heels — after winning a three-setter for the third time in the tournament, 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.

The men’s side features an Olympics replay on hard court. Del Potro beat Djokovic on Wimbledon’s grass to win his bronze, the first of four medals for Argentina.

“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 right-hander 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 was scheduled for the night match.

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.