Reigning US Open champion Sam Stosur prevented another matchup in the Williams sisters' sibling rivalry by beating Venus Williams in three sets Friday at the Family Circle Cup. The second-seeded Stosur won five of the last six games of the match for a 6-3, 4-6, 6-3 victory in the quarterfinals on a chilly, windy Friday at the season's first clay-court event. Stosur now has a chance of knocking out both Williams sisters on consecutive days when she meets Serena in Saturday's semifinals. Serena Williams advanced when opponent Sabine Lisicki twisted her left ankle early in the opening set and had to withdraw after five games. The other semifinal will be between Lucie Safarova and Polona Hercog. The ninth-seeded Czech Safarova beat No. 4 Vera Zvonareva of Russia 6-3, 6-3, while No. 14 Hercog of Slovenia breezed past 13th-ranked Russian Nadia Petrova 6-1, 6-2. The buzz for a Williams' sisters showdown had grown all week as the two Americans moved through the draw. Both came into Friday without losing a set and appeared ready to settle some family business at the Family Circle Tennis Center. "I have come so far, so far from the US Open," she said. Stosur shook off some tired legs from an earlier match – the Australian was forced to come back Friday morning and finish her rain-delayed match from the night before – and some wayward serves midway through to move past Williams. Stosur is seeking her second Family Circle Cup title in three years. Lisicki was having an MRI. — Agencies