This much is certain: When it comes to playing Justine Henin at Grand Slams this year, Williams has been overmatched every time.

Finding all the right angles and hanging tough on long rallies, Henin beat Williams 7-6 (3), 6-1 tonight to reach the U.S. Open semifinals. They have met in the quarterfinals at three consecutive majors, and Henin is 3-0.

Asked if she could explain what went wrong, a sullen Williams replied: "No. I can't. I'm sorry. Any more questions?"

"She made a lot of lucky shots," Williams said a moment later, a white baseball cap pulled low over her eyes, "and I made a lot of errors."

So while the formerly No. 1-ranked Williams is the active leader among women with eight Grand Slam titles and will stay on that number, current No. 1 Henin will have a chance to get her seventh major.

"Playing Serena is really exciting for me," Henin said after compiling a 30-17 edge in winners. "I was really happy about the second set. I played much more aggressive."

Henin's next opponent could be another Williams: Serena's older sister, Venus, faces No. 3 Jelena Jankovic in the quarterfinals Wednesday night. Not much question for whom Henin will be rooting — she's 1-7 against Venus Williams, 7-0 against Jankovic.

Henin beat the younger Williams at the French Open en route to her fourth title in five years there, and again at Wimbledon.

"I got a lot of confidence in Paris and London," Henin said.

Their matchup at Wimbledon in July didn't feature Williams at her absolute best: She could barely hit backhands after spraining her left thumb and hurting her left calf in the previous round. Because of the thumb, Williams withdrew from every event she was scheduled to play in the 1› months between Wimbledon and the U.S. Open.

"I don't think that affected me," she said. "I don't think it did. Maybe it did."

She arrived at Flushing Meadows full of confidence, as usual, and said she didn't need tuneup tournaments to get ready. While Williams' thumb appeared fine, and there were no obvious injuries, her father said Sunday that she was dealing with other physical problems — and doing a "marvelous job of hiding it."

Regardless, Tuesday's match featured brilliant play by both past U.S. Open champions, from stinging serves to big groundstrokes to touch volleys. Henin was far better at the net, winning 11 of 14 points when she pressed forward, while Williams was only 5-for-14.

Mostly, though, the difference-maker was Henin's ability to steer Williams this way and that along the baseline until the Belgian could find an opening for her smooth backhand or a well-placed forehand. Repeatedly, especially in the second set, Henin wrong-footed Williams to end a point.

The first set was much closer and of much higher quality. Henin broke in the opening game by ending a tight net exchange with a forehand volley, and she served for the opening set at 5-4.

But Williams buckled down to break back right there with a backhand return winner down the line and a loud, "Come on!" The tiebreaker began with a 23-stroke point in which Williams made two great defensive plays before swatting a backhand passing winner down the line.

"I really got nervous at that time," Henin said, "and I knew every point was important."

She showed it, delivering a 105 mph ace to go up 4-2, followed by a forehand winner she punctuated with a yell of "Allez!" Williams seemed deflated and dumped a forehand into the net to end the set.

"It's kind of like she had no energy. No get-up-and-go," said Williams' mother and coach, Oracene Price. "Tennis is a head game, sometimes. Your head tells you to do something, but you can't."

Price rested a cheek on a hand, barely able to watch, as Henin took control right away in the second set, breaking for a 2-0 lead, and that was that, pretty much.

As compelling as the story lines were for Henin-Williams, the match turned out to be far less competitive than the men's fourth-round action earlier Tuesday.

Put it this way: A weary Novak Djokovic was relieved to pocket his victory.

The No. 3-seeded Djokovic reached the U.S. Open quarterfinals for the first time by beating No. 23 Juan Monaco 7-5, 7-6 (2), 6-7 (6), 6-1 in a match that included the unusual sight of Monaco losing a point because a ball fell out of his shorts.

"I went nuts! I was cursing at me. I was yelling at my pants," Monaco said.

He was serving while ahead 5-3 in the third-set tiebreaker when the players settled into an extended baseline exchange. As Monaco whipped a shot, an extra yellow ball he had tucked away in case of a fault popped out of his pocket — the second time it happened during the match. He stopped playing, dropped his racket, doubled over and screamed, knowing what was coming.

"Right away I knew I lost that point. It's very weird," Monaco said. "That sort of thing cannot happen twice in the same match."

Chair umpire Jake Garner had no choice.

"By rule," he announced, "he loses the point."

Djokovic gladly took it.

"I never had this situation in match," he said. "First-time experience."

It got Djokovic to 5-4, and he held a match point at 6-5. Monaco saved that with a service winner, and eventually won that set. But Djokovic grabbed a 3-0 lead in the fourth set and wound up winning in 3 hours, 53 minutes.

Djokovic now faces No. 17 Carlos Moya. The 31-year-old Moya is the oldest man still in the tournament, and the player he beat 7-5, 6-2, 6-7 (5), 6-4 Tuesday, 19-year-old Ernests Gulbis of Latvia, was the youngest left.

After Williams and Henin were done, No. 2 Rafael Nadal faced No. 15 David Ferrer. The Nadal-Ferrer winner was to meet No. 20 Juan Ignacio Chela, who got past Stanislas Wawrinka in five sets.

Like Djokovic, Chela reached his first U.S. Open quarterfinal. Moya, however, has been there before — although not since 1998.