Venus Williams, battling a mystery bug and a dogged opponent who refused to lie down, advanced to the third round of the Family Circle Cup in Charleston on Wednesday.

A day after her younger sister Serena was surprisingly beaten at the South Carolina event, Williams kept the family flag flying with a 7-5 7-5 victory over South African Chanelle Scheepers.

It was anything but easy for the 33-year-old Williams, who won the tournament a decade ago but has struggled with health issues in recent years.

She trailled 3-1 in the opening set then 4-1 in the second but fought back each time to seal victory in an hour and 50 minutes.

“She was playing so well, but good thing is that when I get behind, I give even more effort and get even more focused,” Williams told reporters. The next opponent for Williams is rising Canadian star Eugenie Bouchard.

The biggest surprise on Wednesday was the defeat of Sloane Stephens. Seeded fifth, she fell to a 6-4 6-4 loss to Ukrainian teenager Elina Svitolina.

With Serena and now Stephens out of the tournament, just three of the top five seeds remain in the event.

Serbia’s Jelena Jankovic, the second seed and 2007 champion, beat American Lauren Davis 6-0 6-3 while Italian third seed Sara Errani cruised into the third round when she won the first set against Kiki Bertens after her Dutch rival retired.

The fourth seed, Germany’s 2009 champion Sabine Lisicki, had a much tougher time against American Vania King but survived 7-5 4-6 6-3.