The last four women's Grand Slam champions, including world No. 1 Victoria Azarenka and Wimbledon titlist Petra Kvitova, were all on Thursday's second-round schedule at the star- studded $740,000 Porsche Tennis Grand Prix.

The top-seeded Australian Open titlist Azarenka was leading crowd favorite Andrea Petkovic 6-2, 4-4 when the German star retired because of a right ankle injury. "Petko" returned from a back-induced three-month injury layoff just this week.

Azarenka's quarterfinal opponent on Friday will be another German, wild card Mona Barthel, who upended seventh-seeded former Wimbledon runner-up Marion Bartoli, of France. 6-3, 6-1.

A third-seeded Kvitova, meanwhile, drubbed last year's French Open runner-up and 2010 Roland Garros champion Francesca Schiavone 6-2, 6-2 on the indoor red clay at Porsche Arena.

An upset came when rising German Angelique Kerber defeated sixth-seeded former top-ranked star and 2011 Stuttgart runner-up Caroline Wozniacki for the second time in 12 days with a 63-minute 6-1, 6-2 decision. Kerber also surprised the Danish star in a final in Copenhagen two weeks ago.

The former U.S. Open runner-up Wozniacki is still seeking her first title of 2012.

Kerber and Kvitova will clash here on Friday, with Kerber seeking a sixth victory over a top-10 player this year.

Also, fifth-seeded U.S. Open champ Samantha Stosur held off German favorite and defending Stuttgart titlist Julia Goerges 6-2, 2-6, 6-3 and eighth-seeded French Open champ Li Na handled Uzbekistani lucky-loser Akgul Amanmuradova 6-4, 6-4. Goerges became the first German champion here in 17 years (Anke Huber) last year by stunning the then-world No. 1 Wozniacki in the Porsche finale.

Up next for the Australian Stosur will be second-seeded Aussie Open and Wimbledon runner-up Maria Sharapova, while the Chinese Li will encounter rising fourth-seeded Pole Agnieszka Radwanska. Stosur was the runner-up in Stuttgart two years ago.

The 2012 Stuttgart winner will claim $37,000 and a new Porsche sports car.