Defending champion Angelique Kerber will meet Petra Kvitova of the Czech Republic in the Porsche Grand Prix semi-finals, with top-seeded Agnieszka Radwanska taking on qualifier Laura Siegemund.
The second-seeded Kerber easily overcame seventh-seeded Carla Suarez Navarro of Spain 6-2 6-4 in Friday's quarter-finals, with the German 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 quarter-finals 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.
Siegemund upset sixth-seeded Roberta Vinci 6-1 6-4 in 76 minutes to improve on her previous best of a quarter-final 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.
Siegemund next faces top-seeded Radwanska, who defeated Karolina Pliskova of the Czech Republic 6-2 7-6 (10-8) in the last quarter-final.
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