Madison Brengle of the United States upset third-seeded Petra Kvitova 6-3, 7-6 (2) in the second round of the Porsche Grand Prix on Thursday.
Fourth-seeded Caroline Wozniacki won 7-6 (5), 6-1 against Lucie Safarova of the Czech Republic to reach the quarterfinals as well.
Brengle served for the match at 5-4 but Kvitova saved three match points to force a tiebreaker. Brengle took a commanding lead and won her first match over a top-10 player with a backhand passing shot.
Also, Caroline Garcia of France, Ekaterina Makarova of Russian, and Sara Errani of Italy advanced.