Stuttgart, Germany (SportsNetwork.com) - Wimbledon champion Petra Kvitova was a stunning second-round loser, while former world No. 1 Caroline Wozniacki reached the quarterfinals at the Porsche Tennis Grand Prix.

American Madison Brengle doused the third-seeded Czech left-hander Kvitova 6-3, 7-6 (7-2), while the fourth-seeded U.S. Open runner-up Wozniacki got past Czech Lucie Safarova 7-6 (7-5), 6-1 on the indoor clay at Porsche Arena.

Wozniacki was a Stuttgart finalist in 2011.

Meanwhile, sixth-seeded Russian Ekaterina Makarova eased into the quarters by leveling Russian lucky-loser Marina Melnikova 6-2, 6-3.

In some other second-round action, former French Open runner-up Sara Errani handled Kazakhstan's Zarina Diyas 6-4, 6-1 and rising Frenchwoman Caroline Garcia grounded German wild card Carina Witthoeft 7-6 (7-4), 6-2.

The 2015 Stuttgart champ will earn $108,000 and a new Porsche 911 Carrera sports car.