Stuttgart - Defending champion Angelique Kerber dug deep and came from behind to defeat fellow German Annika Beck 4-6, 6-3, 6-1 at the Porsche Grand Prix on Wednesday.
Kerber, seeded second, needed two hours, nine minutes to win the second-round match and reach the quarterfinals.
"I knew that it would be very difficult today and that I'd have to play my best tennis," Kerber said. "Thoughts of my win here last year helped me come back."
Earlier, Karolina Pliskova fired 16 aces to upset eighth-seeded Czech compatriot Lucie Safarova 7-6 (4) 4-6, 7-5. Safarova had 12 aces but also produced nine double-faults in their first-round match.
Sixth-seeded Roberta Vinci of Italy advanced in the indoor clay-court event by outlasting Ekaterina Makarova of Russia 7-6 (3), 4-6, 6-4.
Also in the first round, Andrea Petkovic of Germany beat Kristina Mladenovic of France 6-2, 6-4 and wild card Julia Goerges of Germany breezed past Alize Cornet of France 6-4, 6-0. Monica Niculescu of Romania defeated another Frenchwoman, Caroline Garcia, 6-2, 6-2.