Former champions Serena Williams and Venus Williams were easy third-round winners Thursday at the $740,000 Family Circle Cup tennis event.

The fifth-seeded Serena leveled 17th-seeded Marina Erakovic, of New Zealand, 6-2, 6-2 on the green clay at Family Circle Tennis Center. The American great advanced in 1 hour, 28 minutes without losing her powerful serve on Day 4.

The former world No. 1 Serena titled here in 2008 and was the Family Circle runner-up in 2003. The 2011 U.S. Open runner-up will meet the Sabine Lisicki- Yaroslava Shvedova victor in Friday's quarterfinals.

Meanwhile, the wild card Venus handled Aussie and fellow non-seed Anastasia Rodionova 7-5, 6-2 in 77 minutes. The former world No. 1 smacked six aces and was broken only once en route to victory.

Venus, who titled here in 2004, could face her younger sister in the semifinals this weekend.

Upsets came when 14th-seeded Slovenian Polona Hercog popped seven aces in dousing third-seeded Frenchwoman Marion Bartoli 6-4, 1-6, 6-4 and 13th-seeded Nadia Petrova dismissed eighth-seeded fellow Russian Anastasia Pavlyuchenkova 6-1, 6-4. The 2006 Charleston champion Petrova will face Hercog on Friday.

Ninth-seeded Czech Lucie Safarova reached the round of eight by holding off capable Canadian Aleksandra Wozniak 6-2, 6-7 (4-7), 6-1.

This week's titlist will claim $115,000.