The woman with the best chance to beat Venus Williams at Key Biscayne watched the women's quarterfinals from the photo pit. Serena Williams had no camera but used her connections to secure a front-row seat as she watched her sister beat Radwanska.

Top-ranked Serena has been sidelined by a knee injury since winning the Australian Open in January. In her absence, Venus has become the woman to beat as she bids for her fourth Key Biscayne title and her first since 2001.

"When I'm executing and playing my best, it's great," Venus said. "It feels good, and I feel like I'm definitely dictating the points and that I don't give my opponent as many chances to have a say."

The oldest women's quarterfinalist at 29, Venus has won 14 matches in a row, and two more would give her three consecutive tournament titles for the first time since 2002.

In the men's fourth round, American Mardy Fish retired with a sciatic nerve injury trailing Russia's Mikhail Youzhny 6-1, 1-0. The injury occurred when Fish took a tumble in the first set, causing discomfort from his back to his calf.

No. 8 Jo-Wilfried Tsonga of France earned a shot at Nadal on Wednesday by beating No. 12 Juan Carlos Ferrero of Spain 6-2, 6-2. The fourth-seeded Nadal, seeking his first Key Biscayne championship and his first title anywhere in 10 months, punctuated his win over Ferrer with a nifty leg kick-uppercut combination.

The No. 3-seeded Williams hit eight aces and lost only five points on her first serve. She broke five times, including in the pivotal eighth game, when Radwanska had consecutive double-faults and then hit a 62-mph knuckleball serve that Williams pounced on to whack a winner.

Williams' opponent Thursday will be No. 13 Marion Bartoli of France, who beat No. 12 Yanina Wickmayer of Belgium 6-4, 7-5.