BEIJING, Oct. 3 (Xinhua) -- Serena Williams of the United States was forced to withdraw from the China Open ahead of her quarterfinal match on Friday due to a left knee injury, sending Australian Samantha Stosur into the semifinals.
Williams just rallied for a three-set victory over Lucie Safarova on Thursday night, but she claimed that the knee injury was with her through the match.
"Well, it throbs just sitting, standing. I felt it mostly serving because I'm landing on my left knee. That was really killer for me," she said.
"So as a defending champion, you always want to do your best to defend your title. So more than anything, it's extremely frustrating. You know, I wanted to be here. To come all this way and not to take the title, it's just a waste."
Williams' participation in the WTA Finals to be held in Singapore is in doubt. "If I feel this way for Singapore, I don't think it's smart for me to play. Other than that, I'm just going to see how it goes," she said.
World No. 3 Petra Kvitova of the Czech Republic kept her perfect run in Asia by beating Italian Roberta Vinci in a tight two-setter to reach the semifinals. Her reward will be a semifinal match against Stosur.
Vinci stunned No. 5 seed Agnieszka Radwanska and No.12 seed Ekaterina Makarova in previous rounds and it appeared she was going to keep that string of upsets going as she jumped out to a 5-1 lead against Kvitova.
Point by point and game by game, Kvitova built her comeback, though, coming within two points of dropping the opening set several times, and in the tiebreak she took it to another gear, reeling off six points in a row to lead 6-1 and eventually blasting a big ace to close it out at 7-2.
There was more danger in the second set, but Kvitova kept her cool to claim the set at 6-4.
"I think the start of the match, it happens like this sometimes. She's not really my favorite player to play against," Kvitova said. "She has a really different game that really doesn't suit me. She has a great slice and it gives me a lot of trouble. When I'm not serving, it's just tough to play against her."
Elsewhere, Russian Maria Sharapova enjoyed a comfortable 6-0, 6-4 victory over compatriot Svetlana Kuznetsova to also reach semifinals, where she will compete against Serb Ana Ivanovic.
In men's part, world No. 1 Novak Djokovic from Serbia held off a late fightback from Grigor Dimitrov of Bulgaria as he improved to a 22-0 mark at the China Open with a 6-2, 6-4 victory for the semifinals.
"It was pretty slow conditions because of the weather," said Djokovic. "It's a bit cold, which I think suited more to my style of the game. I was trying to mix up the pace, give him high balls to the backhand, serve efficiently. Everything worked well. The second set was up and down. But generally it was a good performance."
In a blockbuster semifinal, Djokovic will face Scot Andy Murray, who defeated Marin Cilic of Croatia 6-1, 6-4 in another quarterfinal.
It will be the pair's 22nd meeting, with Djokovic leading their head to head series 13-8. The Serb has won both their contests this season.
"Andy just won last week the Shenzhen tournament, he's on a seven-match winning match streak," said Djokovic. "I've seen a little bit of his matches this week. He's hitting the ball very well. Every time I play him, it's a huge challenge. It's a very physical match. A lot of long rallies, exchanges."
In the other semifinal, Slovakia's Martin Klizan will take on Tomas Berdych from the Czech Republic. En route to the semifinals, Klizan upset second seed Rafael Nadal of Spain 6-7 (7),6-4, 6-3 while Berdych defeated US player John Isner 6-1, 6-4.