CHARLESTON, S.C. — Serena Williams advanced to the Family Circle Cup quarterfinals today, overcoming a sprained ankle and sluggish performance to beat Katarina Srebotnik 4-6, 6-4, 6-3.

"I just kept making errors and it wasn't pretty out there," the fifth-seeded Williams said after her more than two-hour match. "It wasn't the best tennis for me."

Williams, seeking her third tournament title this year, will face second-seeeded Maria Sharapova or unseeded Tatiana Perebiynis in the quarterfinals.

Williams, ranked No. 9 in the world, slipped and fell near the net when trailing 4-3 in the first set. She was unsure how the injury may affect her Friday.

"You know when you're playing and the adrenaline, you really can't assess yourself until the next day," she said. "It's kind of frustrating."

Meanwhile, defending champion Jelena Jankovic, the top seed, had an easier time, defeating unseeded Anastasia Rodionova 6-0, 6-2 in less than an hour.

In the quarterfinals, Jankovic will face ninth-seeded Vera Zvonareva, a 6-4, 4-6, 6-1 winner over sixth-seeded Marion Bartoli.

Tenth-seeded Agnes Szavay beat eighth-seeded Dinara Safina 6-2, 7-5, and will face Alize Cornet in the quarterfinals. Cornet beat Sorana Cirstea 6-1, 7-5.

Fourth-seeded Elena Dementieva also advanced, beating 13th-seeded Victoria Azarenka 6-4, 7-5. Dementieva will face No. 7 Patty Schnyder or No. 11 Agnieszka Radwanska.