Victoria Azarenka reached the quarter-finals of the Porsche Grand Prix when Andrea Petkovic retired injured midway through the second set.
Azarenka breezed through the first set 6-2 in Stuttgart 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.
Update:
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