With fans cheering her mistakes, Williams fell short in her bid for a fifth consecutive major title Thursday, losing to Justine Henin-Hardenne 6-2, 4-6, 7-5 in a riveting French Open semifinal.
Williams had won 33 matches in a row at Grand Slam events, a streak that began last year at Roland Garros and was the sixth longest in the Open era.
Williams became tearful after the match when discussing the hostile crowd.
"It's a little difficult," she said. "I've had to fight. It's just another fight I'm going to have to learn how to win. I'm just going to have to keep smiling."
Henin-Hardenne rallied from a 4-2 deficit in the final set and advanced to the first all-Belgian Grand Slam final.
"I have beaten the strongest player in the world," Henin-Hardenne said. "To do it this way, with this atmosphere, clearly it's the most beautiful thing I've had in my career."
On Saturday she'll play Kim Clijsters, who defeated Nadia Petrova 7-5, 6-1 in the other semifinal.
It will be the first Grand Slam final since the 2002 Australian Open featuring someone other than the Williams sisters.
Rooting for Henin-Hardenne from the start, center-court fans cheered even when Williams missed several serves. That was in the seventh game of the final set, when the crowd jeered Williams for questioning two close calls that ultimately went in her favor.
Williams became so rattled by the rough treatment that she lost four consecutive points and her serve, leaving her with a shaky 4-3 lead.
Her mother and coach, Oracene Price, criticized the fans for "a lack of class and total ignorance. Or they just don't know tennis and the etiquette of tennis."
Henin-Hardenne said she was grateful for the fan support but added, "It's true that sometimes it was a little bit too much."
Williams said she didn't think recent tensions between France and United States were a factor in the fans' mood.
"I don't think being American right now had anything to do with it," she said. "I just think sometimes you want the underdog to win."
When Williams hit an errant return on match point, she walked around the net and briskly shook Henin-Hardenne's hand as the crowd booed her. There were more jeers when she headed for the exit.
Said Williams' mother: "She knew this job was dangerous when she took it."
Henin-Hardenne broke for a 5-4 lead in the final set, then had to overcame a case of nerves. Serving for the match, she double-faulted twice and misplayed two groundstrokes to lose the game at love.
"I panicked a little," Henin-Hardenne said.
She regrouped in the next game, and consecutive backhand errors by Williams gave Henin-Hardenne another lead, 6-5. This time she served it out.
"I was very calm because I had just been in the same situation," she said.
Henin-Hardenne hit a perfect lob and two service winners to reach 40-0, and on match point Williams yanked a backhand return wide.
"She's had her chance so many times," Henin-Hardenne said. "Maybe it's time to give someone else a chance."
On Saturday, Henin-Hardenne and compatriot Clijsters will both be bidding for their first Grand Slam title.
"It's unbelievable for the Belgian country," Henin-Hardenne said. "It's an amazing situation."
Henin-Hardenne will be playing in her second Grand Slam final. Venus Williams beat her to win the 2001 Wimbledon title. This time the opponent will be Clijsters, who edged Henin-Hardenne in the 2001 French Open semifinals and was two points from the title before losing the final to Jennifer Capriati.
Henin-Hardenne has long drawn raves for possessing the sport's best backhand, and she can now also claim the honor of being Serena Williams' chief nemesis. Williams was 21-0 this year before losing to Henin-Hardenne at Charleston, S.C., on April 13.
The latest upset was perhaps more surprising, given that Williams had lost only 19 games and no sets in five previous matches at Roland Garros.
Williams was outplayed at the start by Henin-Hardenne, who was more relaxed, moved better and showed more patience. Henin-Hardenne hit aggressively enough to keep the defending champion off balance, and served smartly to neutralize Williams' normally ferocious returns.
"I got off to a slow start," Williams said. "I allowed the crowd to get into the match. When you allow them, it's difficult to stop them."
In the second set, both players held serve four times in a row, then traded three consecutive breaks. Williams cracked a backhand to take the set and even the match. But unlike so many of Williams' opponents, Henin-Hardenne refused to wilt.
In the earlier semifinal, Clijsters lost the first service break to trail 5-4. Facing set point in the next game, she hit a backhand drop shot that clipped the net, then fell on Petrova's side for a winner.
Consecutive backhands into the net by Petrova gave Clijsters the game to make it 5-all, and the turnabout deflated the unseeded Russian. She was broken again two games later to lose the set as Clijsters began to pull away, winning nine of the final 10 games.
"I was a little bit down when she touched the let cord," Petrova said.
"Definitely, I think those things can really turn sets around and matches around," Clijsters said. "I was struggling a little with my forehand and not feeling the rhythm. Once I broke her that first time, I felt comfortable I could do it."
One of the men's semifinals Friday will be an all-Spanish rematch of last year's final, with defending champion and Albert Costa facing No. 3-seeded Juan Carlos Ferrero. Costa is the first player to win four five-set matches in a single French Open.
The other match will be between No. 7 Guillermo Coria and unseeded Martin Verkerk.