Defending champ Andrea Petkovic reaches Charleston quarter-finals

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.