Kerber finished off her 6-1, 6-0 rout of Alexandra Cadantu in 50 minutes, winning two-thirds of the points in the match.
The only game Kerber lost was a service break but she took 77 percent of the points off Cadantu's serve. Cadantu won eight points in her seven games serving.
Dominika Cibulkova, the No. 7 seed, was nearly as dominant in beating Katarzyna Piter 6-0, 6-1. She also won two-thirds of the points and had six breaks in a 53-minute victory.
Second-seeded Sloane Stephens had a much more difficult time in her second-round match, needing 2 1/2 hours to defeat Andrea Petkovic 7-6 (7-1), 4-6, 6-3. Each player had six breaks but Stephens had a 2-1 advantage in third-set breaks to win.
The other matches Thursday were also lengthy with Patricia Mayr-Achleitner defeating Maryna Zanevska 6-1, 6-7 (1-7), 6-4 and Stefanie Voegele getting past Karin Knapp 4-6, 6-2, 7-5.
Friday's quarterfinals send Stephens against Voegele and Cibulkova versus third-seeded Ana Ivanovic in the top half of the draw. Kerber meets Mayr-Achleitner and fourth-seeded Carla Suarez Navarro plays No. 5-seed Kirsten Flipkens in the other half.