Tennis: Rain keeps Williams sisters waiting

World No. 1 Serena Williams and Venus Williams were among the players left waiting on Thursday as rain wreaked havoc at the WTA claycourt tournament in Charleston.

Second-seeded Caroline Wozniacki advanced to the quarter-finals via walkover after Germany's Andrea Petkovic pulled out of their third-round match with a right calf injury, but for much of the day that non-event was the only action.

Four third-round matches were postponed until Friday: top-seeded Serena Williams v Mallory Burdette, fifth-seeded Venus Williams v Varvara Lepchenko, sixth-seeded Lucie Safarova v Sorana Cirstea and Bethanie Mattek-Sands v Madison Keys.

Three matches finally got on Charleston's green clay courts on Thursday night, with 2007 Charleston champ Jelena Jankovic of Serbia booking her quarter-final berth with a 6-0, 6-4 victory over US qualifier Jessica Pegula.

Pegula, whose first- and second-round victories here were the first WTA match wins of her career, seized a 3-1 lead in the second set before former world No. 1 Jankovic roared back.

Unseeded Swiss Stefanie Voegele ousted 10th-seeded German Julia Goerges 3-6, 6-1, 6-3 to set up a quarter-final clash with Wozniacki.

Canadian Eugenie Bouchard also advanced, when third-seeded Australian Sam Stosur retired while trailing in the second set of their match with the same calf injury that prompted her to withdraw from Indian Wells in March and skip the Miami hardcourt tournament.

Bouchard was leading 6-1, 2-0 when former US Open champion Stosur called it a night. The young Canadian will face Jankovic in the quarter-finals.

On Friday, Serena Williams will try to continue her march toward a third Charleston title against Burdette, who ousted 2009 champion Sabine Lisicki of Germany in the second round.

Serena routed Safarova in last year's final, and also won the title in 2008.