Updated

Defending champion Victoria Azarenka has withdrawn from the BNP Paribas Open because of a right ankle injury that had been bothering the world's second-ranked player since before the tournament began.

Azarenka had been scheduled to play No. 8 seed Carolina Wozniacki in a quarterfinal match Thursday afternoon that would have pitted two former top-ranked players against each other.

Azarenka says she was in too much pain while warming up to try to play. She says a doctor advised her to rest because her ankle has inflammation and tendinitis in it.

She's the second player to withdraw on Thursday. Earlier, No. 7 seed Sam Stosur pulled out of her quarterfinal against Angelique Kerber because of a right calf injury.

Kerber and Wozniacki will move into a semifinal against each other.