Charleston (South Carolina): Fifth-seeded Serena Williams overpowered New Zealand's Marina Erakovic 6-2, 6-2 on Thursday to reach the quarterfinals of the WTA's green clay Charleston tournament.

Williams, who had six aces and won 80 percent of her first serve points, next faces either sixth-seeded German Sabine Lisicki or Yaroslava Shvedova of Kazakhstan, with a possible semifinal clash with elder sister Venus Williams looming.

Venus, playing on a wild card invitation as she tries to rebuild a ranking dented by a lengthy injury and illness layoff, booked her quarterfinal berth with a 7-5, 6-2 victory over Australian Anastasia Rodionova.

Venus next faces either second-seeded Samantha Stosur or Kazakhstan's Galina Voskoboeva.

Stosur, the US Open champion ranked fifth in the world, was left as the highest-ranking player in the field after the injury withdrawal on Sunday of Agnieska Radwanska, just a day after her triumph in Miami.

Marion Bartoli, the third seed from France, was toppled 6-4, 1-6, 6-4 by 14th-seeded Polona Hercog of Slovenia.

Bartoli may have been feeling the effects of her second-round victory over Vera Dushevina a day before, which took more than three hours.

Hercog set up a quarterfinal clash with Russian Nadia Petrova, a 6-1, 6-4 winner over eighth-seeded Anastasia Pavlyuchenkova of Russia.