Charleston, SC (SportsNetwork.com) - Reigning champion Andrea Petkovic and rising American star Madison Keys were a pair of easy third-round victors Thursday at the Family Circle Cup tennis event.

The third-seeded world No. 11 Petkovic handled American Madison Brengle 6-4, 6-4 on the clay courts at Family Circle Tennis Center.

The sweet-swinging German Petkovic defeated Slovak Jana Cepelova in last year's Family Circle finale. Her quarterfinal opponent on Friday will be Montenegro's Danka Kovinic, a qualifier who received a walkover into the round of eight when sixth-seeded former world No. 1 Jelena Jankovic pulled out of their scheduled Thursday night match.

The Serbian star Jankovic titled here in 2007 and was the runner-up in 2013.

The seventh-seeded Illinois native and Australian Open semifinalist Keys cruised past Romanian Andreea Mitu 6-2, 6-0 to set-up a quarterfinal match with fellow American Lauren Davis.

Meanwhile, fourth-seeded former French Open runner-up Sara Errani held off Spanish qualifier Sara Sorribes Tormo 6-2, 5-7, 6-2 and fifth-seeded German left-hander Angelique Kerber spanked Spaniard Lara Arruabarrena 6-3, 6-0.

Up next for the gritty Italian Errani will be Czech Lucie Hradecka, while Kerber's quarterfinal opponent will be 13th-seeded Romanian Irina-Camelia Begu, who received a walkover into the round of eight when second-seeded Russian Ekaterina Makarova pulled out of the draw on Wednesday due to an illness.

In other action involving seeds, Hradecka overcame No. 8 Frenchwoman Caroline Garcia, a recent back-to-back runner-up at a pair of events in Mexico, 5-7, 7-5, 6-4, and the surprising Davis upended No. 15 German Mona Barthel 6-4, 3-0, retired. The 21-year-old Ohio native Davis stunned top-seeded Canadian star Genie Bouchard in straight sets here on Wednesday.

"I love this tournament," Davis said. "It's an amazing tournament. I love the support. And I feel really comfortable on the clay here -- I feel like I'm sliding well and that my game really suits it well."