Defending champion Victoria Azarenka withdrew from the BNP Paribas Open on Thursday because of a right ankle injury that had been bothering the world's second-ranked player since before the tournament began.
Azarenka was scheduled to play No. 8 seed Caroline Wozniacki in an afternoon quarterfinal that would have pitted two former top-ranked players against each other.
''It's already really painful, but it's very high risk to make it much worse,'' Azarenka said.
Earlier, No. 7 seed Sam Stosur pulled out of her quarterfinal against No. 4 Angelique Kerber because of a right calf injury.
Wozniacki and Kerber will move into a semifinal against each other. No. 2 Maria Sharapova and No. 13 Maria Kirilenko will play in the other semi.
Azarenka had been in obvious pain during her earlier matches, but she downplayed it when asked. She said her ankle has been bothering her since before she played Serena Williams in a New York exhibition on March 5.
''It's just worse,'' she said. ''I tried not to practice yesterday, and to see if it will settle down at least a little bit. I felt yesterday was better, not walking in my tennis shoes. But today, once I started doing the exercise, that was it.''
Azarenka said a doctor advised her to rest because her ankle has inflammation and tendinitis in it. She added that she plans to travel to Key Biscayne, Fla., for next week's tournament.
''The doctor said it might heal quick, the pain might go down quickly because of the inflammation going down, but the healing process is a little bit unknown,'' she said. ''So that's frustrating for me as a player, not knowing.''
The withdrawals left just one men's match on the afternoon schedule a day after a marathon session that ended just before 2 a.m. PDT, with four of the six matches on the main stadium court going to three sets. No. 6 Tomas Berdych played Kevin Anderson in the quarterfinals Thursday.
Defending men's champion Roger Federer and Rafael Nadal were set to play their quarterfinal at night in the earliest matchup between the two rivals since the first time they played each other in 2004. Typically, they don't meet until the semifinals or finals.
Stosur said she hurt her calf muscle in the last game of a three-set win against Mona Barthel on Tuesday. The Australian didn't practice on Wednesday and iced her leg. Stosur was five minutes into her warm-up on Thursday when she realized she couldn't play.
She said this is only the second time she's pulled out of a match in her career and isn't sure how long she'll be sidelined.
