Former world No. 1 Maria Sharapova reached the quarterfinals, while sixth-seeded former top-ranked star Caroline Wozniacki posted a first-round victory Wednesday at the $740,000 Porsche Tennis Grand Prix.

The second-seeded Sharapova, who was this year's Australian Open and last year's Wimbledon runner-up, was leading Alize Cornet 6-3, 1-0 when the French qualifier retired from their second-round match, citing a right shoulder injury.

The sixth-seeded Danish Wozniacki was leading fellow former No. 1 and fellow former U.S. Open runner-up Jelena Jankovic by an identical 6-3, 1-0 score when the Serb retired on Day 3 due to a lower back injury. Jankovic was the Stuttgart titlist back in 2008.

Up next for the 2011 Stuttgart runner-up Wozniacki will be German Angelique Kerber, who stunned the Dane in a final in Copenhagen two weeks ago.

Fourth-seeded rising Pole Agnieszka Radwanska joined Sharapova in the quarters by handling Hungarian qualifier Greta Arn 6-3, 6-4 on the indoor red clay at Porsche Arena.

In some other first-round action, German wild card Mona Barthel doused former world No. 1 Serb Ana Ivanovic 7-5, 7-6 (7-4) and Uzbekistani lucky-loser Akgul Amanmuradova edged out Slovak Dominika Cibulkova 6-2, 5-7, 6-3. Amanmuradova, who replaced Slovak Daniela Hantuchova in the draw, will meet eighth- seeded French Open champ Li Na of China on Thursday.

Also on Thursday, reigning world No. 1 Belarusian star Victoria Azarenka will battle German crowd favorite Andrea Petkovic, third-seeded Wimbledon champion Petra Kvitova will encounter former French Open champ and 2011 Roland Garros runner-up Francesca Schiavone, and fifth-seeded U.S. Open titlist Samantha Stosur will tangle with reigning Stuttgart champ Julia Goerges, who upended Wozniacki in last year's finale here. Stosur was the Porsche Grand Prix runner-up two years ago.

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