Nuremberg - Second-seeded Angelique Kerber came from 5-1 down in the second set to beat Klara Koukalova of the Czech Republic 6-1, 7-6 (2) on Tuesday and reach the quarter-finals of the Nuremberg Cup.
The German next faces Japanese qualifier Misaki Doi, who rallied from a set down to beat Zhang Shuai of China 1-6, 6-3, 6-1 in the second round.
Italians Roberta Vinci and Karin Knapp advanced with comfortable victories.
Fourth-seeded Vinci defeated Czech player Tereza Smitkova 6-2, 6-1, and Knapp, the No. 6, prevailed over German wild card Anna-Lena Friedsam 6-1, 6-3.
Vinci now plays seventh-seeded Kurumi Nara of Japan, who defeated Switzerland's Stephanie Voegele 6-3, 6-4, while Knapp will take on either Kiki Bertens of the Netherlands or Julia Putintseva of Kazakhstan.