Angelique Kerber has advanced to the semifinals of the BNP Paribas Open after quarterfinal opponent Samantha Stosur withdrew because of a right calf injury.

Stosur was hurt during her fourth-round win over Mona Barthel.

"I hurt my right calf muscle in the last game of my match against Mona Barthel when I was serving for the match, so I don't know if you can get any more unlucky than that," Stosur said Thursday morning.

"It's really disappointing. I love the conditions here at the BNP Paribas Open and I love playing in front of this crowd, so to have to pull out without even being able to get on court is probably as bad as it gets. I think this is only the second time I've ever pulled out of a match in my career, and unfortunately there isn't anything I can do about it."

The fourth-seeded Kerber will next play the winner of Thursday's other scheduled quarterfinal between top-seeded Victoria Azarenka and former world No. 1 Caroline Wozniacki.

Kerber will play in her second semifinal of 2013, having lost to Dominika Cibulkova in the Sydney semis back in January.

Maria Sharapova and Maria Kirilenko have already advanced to the final four, winning quarterfinal matches on Wednesday. Sharapova, the 2006 Indian Wells champ and last year's runner-up to Azarenka, topped Sara Errani in straight sets, while Kirilenko dumped fifth-seeded Czech Petra Kvitova in three.

This Premier Mandatory WTA event will pay its newest champion $1 million.