Charleston - Defending champion Andrea Petkovic booked her quarter-final berth at the WTA tournament in Charleston on Thursday, improving her perfect record in the event with a 6-4, 6-4 win over Madison Brengle.
The third-seeded German improved to 10-0 on the green clay of Charleston, avenging her first-round loss to Brengle at the Australian Open.
Petkovic, ranked 11th in the world, is the highest-seeded player left in the draw after the second-round exit of top-seeded Canadian Eugenie Bouchard and the withdrawal of second-seeded Ekaterina Makarova with a stomach ailment.
Petkovic will next face qualifier Danka Kovinic of Montenegro, who reached the quarters when former champion and sixth seed Jelena Jankovic withdrew from their scheduled third-round match with a right foot injury.
Lauren Davis, who stunned Bouchard in straight sets on Wednesday, moved into the quarter-finals with an abbreviated victory over ailing Mona Barthel.
Davis was leading the 15th-seeded German 6-4, 3-0 when Barthel retired because of dizziness.
Although she hasn't spent all that much time on court, the 66th ranked Davis said she was finding her feet on Charleston's clay courts.
"I feel really comfortable on the clay here," she said. "I feel like I'm sliding well and that my game really suits it well."
Davis next faces seventh-seeded American Madison Keys. Keys, an Australian Open semi-finalist, defeated Romanian Andreea Mitu 6-2, 6-0.
Davis and Keys are familiar foes. With their career head-to-head record tied at two victories apiece.
"We know each other really well," Davis said. "For about a year and a half we shared the same coach ... I think we know each other's games like the back of our hands, so it will all come down to who's more competitive."
Fourth-seeded Italian Sara Errani reached the quarter-finals with a 6-2, 5-7, 6-2 victory over Spain's Sara Sorribes Tormo.
Errani will face Czech Lucie Hradecka, who toppled French eighth seed Caroline Garcia 5-7, 7-5, 6-4.
Fifth-seeded German Angelique Kerber eased past Spain's Lara Arruabarrena 6-3, 6-0 to line up a quarter-final meeting with Romania Irina-Camelia Begu. Begu advanced by walkover thanks to Makarova's decision to pull out.Results on Thursday in the WTA claycourt tournament at Charleston (x-denotes seeding):
Lauren Davis (USA) bt Mona Barthel (GER x15) 6-4, 3-0 - retired
Madison Keys (USA x7) bt Andreea Mitu (ROM) 6-2, 6-0
Sara Errani (ITA x4) bt Sara Sorribes (ESP) 6-2, 5-7, 6-2
Lucie Hradecka (CZE) bt Caroline Garcia (FRA x8) 5-7, 7-5, 6-4
Danka Kovinic (MNE) bt Jelena Jankovic (SRB x6) retired
Andrea Petkovic (GER x3) bt Madison Brengle (USA) 6-4, 6-4
Angelique Kerber (GER x5) bt Lara Arruabarrena (ESP) 6-3, 6-0
Irina-Camilia Begu (ROM x13) bt Ekaterina Makarova (RUS x2) walkover