Former champion Serena Williams was an easy quarterfinal winner Friday, while 2010 winner Samantha Stosur drove out Venus Williams at the $740,000 Family Circle Cup tennis event.

The fifth-seeded Serena was handling sixth seed Sabine Lisicki, 4-1 in the first set, when the German retired on the green clay at Family Circle Tennis Center. The former world No. 1 Serena titled here in 2008 and was the Family Circle runner-up in 2003.

Lisicki twisted her ankle and fell to the court while battling Serena early on. She received treatment for the injury and played three more games before quitting in tears.

The capable German was the Charleston champ in 2009.

Serena will meet the second-seeded Stosur, and not her big sister Venus, in the final four, as the Aussie outlasted the 2004 Charleston champ Venus 6-3, 4-6, 6-3. Stosur had to win two matches on Friday in order to reach the semis.

The world No. 10 Serena lost to Stosur in last year's U.S. Open finale, but avenged that setback with a victory in Miami last week.

Stosur had to complete a rain-suspended third-round match from Thursday, which she did with a 6-3, 4-6, 6-2 victory over Kazakhstan's Galina Voskoboeva on Day 5. Stosur led 4-2 in the first set when play was called on Thursday.

This week's winner will earn $115,000.