CHARLESTON, S.C. - Defending champion Andrea Petkovic kept up her perfect run at the Family Circle, improving to 10-0 and advancing to the quarterfinals with a 6-4, 6-4 victory over American Madison Brengle on Thursday.

The third-seeded Petkovic is the highest-ranked player left after No. 1 seed Eugenie Bouchard lost Wednesday night only hours after No. 2 seed Ekaterina Makarova withdrew with a stomach illness.

Petkovic had lost to Brengle in their only other meeting, a first-round matchup at the Australian Open. Petkovic, ranked 11th, was a player on the rise expecting bigger things when Brengle handed her a three-set loss three months ago.

Petkovic now says that loss was a wake-up call, showing her she didn't have a "Plan B" for when she wasn't feeling strong, dealing with harsh conditions or just not in the right mental place to compete.

She spent the past few months working on that. The results were apparent this time against Brengle.

Petkovic trailed 2-1 in each set, digging in both times to rally past her opponent. When Petkovic let her 5-2 lead in the second set slip to 5-4 and Brengle was a point away from tying the set, Petkovic had an overhead putaway to force deuce, then pressured Brengle with deep, precise ground strokes she could not keep in play to take the victory.

"Even though I didn't think I played well today, I played strategically well and that paid off in the end," she said.

Petkovic will play qualifier Danka Kovinic in Friday's quarterfinals. Kovinic advanced when No. 6 seed Jelena Jankovic withdrew Thursday with a right foot injury.

Jankovic, the 2007 Family Circle champion, felt pain in the ball of her foot mid-match Tuesday. When it did not improve, she decided to stop playing. Jankovic will have an MRI on Friday to see the extent of the injury.

Jankovic apologized for having to pull out to fans assembling at Billie Jean King Court, the tournament's featured venue.

"I hope to see you next year," she said.

American Lauren Davis, who ousted Bouchard, advanced into the quarterfinals after No. 15 seed Mona Barthel retired while trailing 6-4, 3-0. Davis fell behind in the match 4-0 before winning nine straight games to reach the Family Circle quarterfinals for the first time.

"I feel really comfortable on the clay," said the 21-year-old from Gates Mills, Ohio. "I feel like I'm sliding well and that my game really suits it well, so yeah, I'm just really enjoying myself."

Davis next plays fellow American Madison Keys, the seventh seed who beat Andreea Mitu, 6-2, 6-0. It's a matchup of two rising U.S. 20-somethings who used to train under the same coach.

Davis said she knew Keys' game like the back of her hand. "Okay," Keys playfully replied. "I guess she's knows everything, doesn't she? She has me figured out."

No. 4 seed Sara Errani topped Sara Sorribes Tormo 6-2, 5-7, 6-2. Fifth-seeded Angelique Kerber also advanced with a 6-3, 6-0 win over Lara Arruabarrena.

Errani will play Lucie Hradecka, a qualifier who beat No. 8 seed Caroline Garcia, 6-7, 7-5, 6-4.

Kerber will meet 13th-seeded Irina-Camelia Begu in the quarterfinals. Begu moved on when Makarova withdrew.

Petkovic, 27, typically avoided the Family Circle, one of the season's early clay-court tournaments, to get some practice for the European events ahead. She decided to give the circuit's oldest women's-only tournament a try two years ago - and hasn't lost since.

Petkovic won two matches here in 2013 before withdrawing due to injury. Last year, she was a surprise champion after two-time defending winner Serena Williams was knocked out early. And Petkovic has continued to thrive this time.

She said the surface suits her game, but she also revels in the easy, relaxed atmosphere in one of South Carolina's coastal gems. The laid-back approach lets Petkovic remember that you don't always have to grind things out or worry about upping the speed of your serve.

"Most you know me and know I'm very uptight. I'm a very stiff German most of the time," she joked.