Angelique Kerber faces Petra Kvitova in Porsche Grand Prix semifinals

STUTTGART, Germany (AP) — Defending champion Angelique Kerber will face Petra Kvitova of the Czech Republic in the Porsche Grand Prix semifinals.

The second-seeded German easily overcame seventh-seeded Carla Suarez Navarro of Spain 6-2, 6-4 in Friday's quarterfinals, needing just 29 minutes to win the first set and 72 minutes overall at the indoor clay-court tournament.

"I gathered a lot of self-confidence through winning the Australian Open (in January) and the other successes," said Kerber, who lost to Suarez Navarro in the quarterfinals in Stuttgart two years ago. "I really wanted revenge today."

Fifth-seeded Kvitova had a tougher task defeating third-seeded Garbine Muguruza of Spain 6-1, 3-6, 6-0.

Qualifier Laura Siegemund upset sixth-seeded Roberta Vinci 6-1, 6-4 in 76 minutes to improve on her previous best of a quarterfinal place in Charleston this month. The 28-year-old German had already upset fourth-seeded Simona Halep on Thursday and is set to improve on her career-high No. 71 ranking.

"I was pretty tired in the second set but you carried me through," Siegemund told her hometown crowd.

Later Friday, top-seeded Agnieszka Radwanska of Poland and Karolina Pliskova of the Czech Republic played the last quarterfinal.