Serena Williams had to dig deep to go through in Charleston

Serena Williams will face Maria Sharapova in the Family Circle Cup last eight after overcoming a sprained ankle to beat Katarina Srebotnik.

The younger of the Williams sisters, seeded five, was below her best but battled to a 4-6 6-4 6-3 victory.

"I just kept making errors and it wasn't pretty out there," she said.

Second seed Sharapova later beat Taiana Perebiynis of the Ukraine 7-5 6-2 while top seed Jelena Jankovic brushed past Anastasia Rodionova 6-0 6-2.

Elena Dementieva beat Victoria Azarenka 6-4 7-5, while both Vera Zvonareva and Agnes Szavay beat higher seeded players.

The former downed Marion Bartoli 6-4 4-6 6-1, with the latter ousting Dinara Safina 6-2 7-5.

Sharapova is chasing her fourth title of the year and Williams said she was unsure how her injury might affect her when the two meet in the quarter-finals on Friday.

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