Stuttgart, Germany (SportsNetwork.com) - 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.
Kerber improved to 3-4 lifetime against the five-time Grand Slam champ Sharapova, who beat the German in a semifinal here two years ago. Kerber has now won their last two matchups.
The reigning French Open titlist and Australian Open runner-up Sharapova captured a third straight Porsche title last year by beating fellow former No. 1 Ana Ivanovic in a marquee finale.
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.
Brengle served for the match at 5-4 against Kvitova, but the Czech slugger saved three match points to force a tiebreak. The Delaware native Brengle eventually won her first match over a top-10 player with a backhand passing shot.
"I think that Madison played a solid match. She didn't miss much and it was really difficult for me because when I had the opportunity for winners I missed," Kvitova said. "The confidence wasn't really high then and I didn't feel physically there as well.
Wozniacki was a Stuttgart finalist in 2011.
"It was a tough first round but I'm happy to be through," Wozniacki said. "I'm pleased with the way I managed to mix up the pace, and I ran a lot of balls down, which was good. Here the clay is a little bit more slippery, so it was all about being the first one to move your opponent, and I did that well."
Sixth-seeded Russian Ekaterina Makarova eased into the quarters by leveling Russian lucky-loser Marina Melnikova 6-2, 6-3, while Thursday's nightcap saw eighth-seeded Spaniard Carla Suarez Navarro top Swiss teenager Belinda Bencic 6-4, 7-6 (7-1).
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.
Friday's quarterfinals will pit a second-seeded Halep against Errani; Wozniacki versus Suarez Navarro; Makarova against Kerber; and Brengle versus Garcia.
The 2015 Stuttgart champ will earn $108,000 and a new Porsche 911 Carrera sports car.