Serena Williams raised her arms so high after her 4-6, 6-3, 6-2 quarterfinal victory over a resurgent Monica Seles tonight that it looked as though she might keep rising if she only stretched a little more. Then again, if she continues to play as she has in this U.S. Open, she just might.

Williams has looked stronger and stronger throughout this tournament, and tonight was no exception as she advanced to her first Grand Slam semifinal. There she will meet defending champion Lindsay Davenport, who showed her first signs of struggle during a 6-2, 3-6, 7-5 squeaker against No. 5 seed Mary Pierce.

Earlier in the day, No. 3 seed Yevgeny Kafelnikov had won an even tighter quarterfinal over Richard Krajicek, who set a record with 48 aces despite losing, 7-6 (7-0), 7-6 (7-4), 3-6, 1-6, 7-6 (7-5). No. 2 seed Andre Agassi finished the action with a 7-6 (7-3), 6-3, 6-4 win over French qualifier Nicolas Escude, but by the second set of his match much of the overflowing -- and star-studded -- crowd had left Arthur Ashe Stadium.

Stars such as musicians Keith Richards and Harry Connick Jr. and filmmaker Spike Lee already had seen who they came to see: an electrifying Serena Williams.

"This is great -- I've been waiting a long time for this," Williams said before practically leaping off the court to the locker room. "I was a little nervous when I first came out, so I put too much pressure on myself, but then in the second set, I completely settled down. And I had 15 aces -- wow!"

About a half-hour later, the 17-year-old Williams had calmed down a little bit -- "believe it or not, it's already settled in," she said -- although she was still giddy enough to spend her post-match interviews playing with her new Jack Russell terrier puppy, Jackie.

The display seemed to be the release of the enormous pressure that has been sitting on Williams since her father, Richard, predicted that she and her older sister, Venus, would play each other in the final of this tournament. Venus did her part Tuesday by advancing to the semifinals in her half of the draw, but Serena had the tougher quarterfinal match.

While still not in the form that won her eight Grand Slam titles before being stabbed by a deranged fan in 1993, Seles has looked extremely sharp in this tournament, and she took the first set tonight as Williams had trouble adjusting to the enormity of the occasion. But by the first game of the second set, Williams's concentration had returned.

She leaped out to a 4-0 lead and had two break points at 4-1. Seles made a bid to get back into the set by tightening the score to 4-3, but Williams broke her yet again in the next game with a punishing overhead. The point spelled the end of the set and, eventually, the end of Seles, who appears to be at a crossroads in her career.

With her rhythm and her strokes finally back in a groove, Seles is now trying to figure out if she has the will to improve her fitness.

"I think it's obvious -- I have my strokes and everything is there, but I just can't physically cover the courts as well," Seles said. "I have my own battles with that, and I have to figure it out in my own brain. I think you have to overcome different things in your life. I've overcome certain things, and certain things I haven't."

While Seles felt she was never quite physically equipped to handle Williams, Pierce got much closer to dethroning Davenport, who had not dropped a set here before today. Pierce held match point twice before nerves -- and a 67-minute rain delay -- finally caught up with her. After the break, the players came back with the third set even at 5, but Davenport was the stronger player, piling up two match points.

On the second, anxiety caught up again with Pierce, who kept taking deep breaths to try to stop her hands from shaking as she tossed the ball up for her serves.

It didn't work. She double-faulted to hand Davenport the match.

"I'm human -- I got a little nervous and a little tight," said Pierce, who wore a devastated stare when packing up her bags after the match but seemed much more serene about an hour later. "You just sometimes want it so bad, you put a little extra pressure on yourself instead of playing the big points like any other point."

Davenport was equally composed after the match, aware of how fortunate she was to still be in a position to defend her title. She said that during the rain delay she just tried to stay calm and think about basic strategies like "hold your serve."

She lay down in the locker room, resting and watching a replay of part of Todd Martin's thrilling fourth-round come-from-behind victory over Greg Rusedski.

"You just go through this so many times that you lose close ones and you win close ones -- it just goes in streaks I think," Davenport said. "I'm just lucky to be here, and you have to just go with that.

"I should have been back on my way home when the rain came. I should have been on the way to Manhattan and starting to pack to go home tomorrow."

U.S. Open

When: Through Sunday.

Where: USTA National Tennis Center, Flushing Meadows, N.Y.

TV: USA network, 11 a.m., 7:30 p.m.

Defending champions: Patrick Rafter, Lindsay Davenport.

Top seeds: Pete Sampras, Martina Hingis.

Yesterday's results: Women's quarterfinals -- Lindsay Davenport (2), United States, def. Mary Pierce (5), France, 6-2, 3-6, 7-5; Serena Williams (7), United States, def. Monica Seles (4), United States, 4-6, 6-3, 6-2.

Men's quarterfinals -- Yevgeny Kafelnikov (3), Russia, def. Richard Krajicek (12), Netherlands, 7-6 (7-0), 7-6 (7-4), 3-6, 1-6, 7-6 (7-5); Andre Agassi (2), United States, def. Nicolas Escude, France, 7-6 (7-3), 6-3, 6-4.

Today's featured matches: Men's quarterfinals -- Gustavo Kuerten (5), Brazil, vs. Cedric Pioline, France; Slava Dosedel, Czech Republic, vs. Todd Martin (7), United States.