STUTTGART, Germany (AP) Wild card Mona Barthel of Germany used her big serve to upset seventh-seeded Marion Bartoli of France 6-3, 6-1 and reach the quarterfinals of the Porsche Grand Prix on Thursday.

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.