By Erik Gudris | Friday, October 3, 2014

World No. 1 and defending champion Serena Williams withdrew with a left knee injury before her quarterfinal at the China Open in Beijing.

Williams, who played with protective tape over her knee in her last match versus Lucie Safarova, said she felt pain all week during a press conference.

"I started to feel pain this week," Williams said. "It didn't feel really well last night at all. I almost didn't go on the court. I did at the last minute. I think it made it a little worse, to be honest."

Williams, scheduled to compete at the upcoming WTA Finals in Singapore later this month will now head to Europe for evaluation. Sam Stosur, Williams's quarterfinal opponent, advanced by walkover into the semifinals.

Kvitova, Sharapova Advance Into Semis

Two women left in the draw have an opportunity to become World No. 2 should either win the Beijing title.

Petra Kvitova fell behind 1-5 in the first set to her opponent Italy's Roberta Vinci. But Kvitova mounted a strong comeback to eventually seal a 7-6(2), 6-4 win.

Kvitova next faces Stosur in the semis with the Czech leading the head to head 6-1.

Maria Sharapova won all-Russian quarterfinal against Svetlana Kuznetsova. Sharapova booked her place in the final four with a 6-0, 6-4 win.

Sharapova will now face Ana Ivanovic.

If either Kvitova or Sharapova win the China Open title, they are projected to climb to No. 2 in the WTA rankings.