Serena Williams had no camera but used her connections to secure a front-row seat as she watched sister Venus beat Agnieszka Radwanska 6-3, 6-1.
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 is the hottest player on the WTA Tour. She 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 an injury trailing Mikhail Youzhny 6-1, 1-0. No. 8 Jo-Wilfried Tsonga beat No. 12 Juan Carlos Ferrero 6-2, 6-2, and No. 33 Nicolas Almagro eliminated No. 27 Thomaz Bellucci 6-4, 3-6, 7-6 (3).
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 struggled with her backhand but won 15 of 19 points at the net.
"I play my best game when I'm more aggressive, because that's my style," she said. "Others don't play as well as aggressive, because that's not their style. It just seems to be better when I move forward, and I try to do that."
Radwanska, seeded sixth, fell to 4-18 against top-five players.