STUTTGART, Germany (AP) -- Top-ranked Victoria Azarenka of Belarus advanced to the Porsche Grand Prix quarterfinals when Andrea Petkovic was forced to retire after injuring her right ankle on Thursday.

Azarenka was leading 6-2, 4-4 when the German stepped awkwardly and twisted her right ankle on the indoor red clay court. Petkovic didn't even try to continue playing and had to be helped off the court.

Ranked No. 11 in the world, Petkovic was in the second match of her comeback after injuring her lower back in January. Petkovic was the first German woman to finish a year in the top 10 since Steffi Graf in 1998.

Angelique Kerber of Germany routed sixth-seeded Caroline Wozniacki of Denmark 6-1, 6-2. Kerber also beat the former world No. 1 to win the title in Copehnagen this month.

Wild card Mona Barthel of Germany used her big serve to upset seventh-seeded Marion Bartoli of France 6-3, 6-1 for a place in the last eight.

Barthel has jumped from No. 196 to 35 in the past year. She had seven aces against Bartoli and dominated with her powerful game.

Wimbledon champion Petra Kvitova of the Czech Republic, the third seed, beat Francesca Schiavone of Italy 6-2, 6-2.

Fifth-seeded Sam Stosur of Australia downed defending champion Julia Goerges 6-2, 2-6, 6-3 for her first win against the German in three attempts.

Li Na of China topped Agkul Amanmuradova of Uzbekistan 6-4, 6-4.

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