Three-time defending Stuttgart champion Maria Sharapova and Wimbledon titlist Petra Kvitova were a pair of second-round losers, while former world No. 1 Caroline Wozniacki reached the quarterfinals at the Porsche Tennis Grand Prix.
German left-hander Angelique Kerber upended the top-seeded former world No. 1 star Sharapova 2-6, 7-5, 6-1. The loss means Sharapova will be supplanted by Simona Halep as the new world No. 2 when next week's rankings are released.
Meanwhile, American Madison Brengle doused the third-seeded Czech left-hander Kvitova 6-3, 7-6 (7-2) and 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.
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.