Serena Williams demonstrated that her shaky knee is holding up fine, sweeping past Patty Schnyder 6-4, 6-2 yesterday to reach the quarterfinals of the U.S. Open.

Once again dressed in black, the two-time Open champion had little trouble in winning her fourth straight match at Flushing Meadows. Williams took this one with her serving, finishing with 12 aces compared with one double fault.

Williams was forced to pull out of last month’s Olympics because of pain in her left knee. She had surgery on it in August 2003, and missed 8-1/2 months while recovering.

In fact, she said a few doctors advised her to skip this tournament.

“My knee is doing pretty good right now. I haven’t had any worries,” she said. “I’m just excited to be here because I wasn’t supposed to come.”

Williams advanced to yet another matchup with Jennifer Capriati, who beat Ai Sugiyama 7-5, 6-2.

Williams is 10-6 lifetime against Capriati, including 3-3 in Grand Slam events. Capriati defeated her in the French Open quarterfinals this year, then lost to Williams in the Wimbledon quarters.

“I like playing Jennifer a lot, I really do,” Williams said. “We always play each other. We really know each other’s games.”

Of Williams, Capriati said, “I’ve played her at her best. I’ve played her at her worst. I’ve played her at my best, me at my worst. I can just only worry about myself, really.”