World number one Victoria Azarenka reached the quarter-finals of the Porsche Grand Prix in Stuttgart when German opponent Andrea Petkovic retired injured midway through the second set of their match.
Azarenka breezed through the first set 6-2 but had been broken twice in the second, which stood at 4-4 when home favourite Petkovic was forced to hand the Belarusian a place in the last eight.
Australian fifth seed Sam Stosur beat defending champion Julia Goerges 6-2 2-6 6-3 in exactly 100 minutes, while third seed Petra Kvitova beat former French Open champion Francesca Schiavone 6-2 6-2.
German pair Angelique Kerber and Mona Barthel pulled off notable wins over sixth seed Caroline Wozniacki and seventh seed Marion Bartoli respectively, while Chinese eighth seed Na Li also progressed.
