The Russian was down but not out. Showing newfound resilience, she squandered a big lead in the second set Wednesday but ended Williams' 18-match Grand Slam winning streak in the French Open quarterfinals, 7-6 (3), 5-7, 7-5.
Williams, seeded second, was seeking her third successive major title and the 11th of her career.
Williams was on the verge of reaching the women's final four after mounting a comeback and taking a 3-1 lead in the final set. She blamed nerves for her defeat.
"In the third set I had an opportunity and I got really tight, and I pretty much gave it to her," Williams said. "It was like, 'Here, do you want to go to the semis? Because I don't.' She was like, 'OK.'"
The No. 7-seeded Kuznetsova faced a set point serving at 5-6 in the first set but erased it with a slam and took the lead. After Kuznetsova's tumble in the second set, Williams erased a 5-3 deficit and rallied three points from defeat to even the match.
"It was very confusing," Kuznetsova said. "Yes, I missed my moments, but she also was playing good."
Kuznetsova mounted the final surge and overcame her history of shaky play when trying to close out big matches. Instead it was Williams who succumbed to the tension.
Why the nerves?
"I don't know," Williams said. "I haven't gotten tight since 2007 in Australia. Maybe I put some expectations on myself that I didn't put on myself initially."
Williams won her only French Open title in 2002.
Kuznetsova, the 2004 U.S. Open champion, seeks her second major title. Her opponent Thursday will be Samantha Stosur of Australia, who beat Sorana Cirstea of Romania 6-1, 6-3.
Stosur, 25, has never won a tour title.
Kuznetsova was serving one point from a 5-2 lead in the second set when she turned her right ankle in pursuit of a shot and fell on her back. She rose caked with clay -- it was even in her hair and on her forehead.
She was unhurt and play quickly resumed, but it took a while for Kuznetsova to regain her footing. Later she could smile about the spill.
"See the sand?" she said at her postmatch news conference. "I still have clay in my hair."
After two sets, the match, which lasted nearly three hours, mirrored the quarterfinals at the Australian Open in January, when Kuznetsova served for the victory at 5-3 in the second set before Williams rallied.
In that case, Williams went on to the title, but this time Kuznetsova recovered from her second-set lapse. She failed to convert two match points in the 10th game of the last set but smacked three service winners to hold for 6-5. A weary Williams committed three unforced errors serving in the final game, including a backhand pushed wide on match point.
The seesaw struggle was this close: Midway through the third set, each player had won 100 points.
"Honestly, I think I lost because of me, and not because of anything she did," Williams said.
Not entirely. High-risk tennis paid off for Kuznetsova, who whacked winners into both corners and sometimes chose angles that surprised Williams. Kuznetsova showed lots of variety, too -- when she hit a cross-court lob, Williams staggered helplessly as she watched the ball land beyond her reach.
"We both fight hard, and it was very interesting match to watch," Kuznetsova said. "I give my best. I was lucky and I won this match. I push myself to the limit."
Stosur reached her first major semifinal. Against Cirstea, Stosur won 92 percent of the points on her first serve in the first set. In the second, Stosur saved eight of the nine break points she faced.
"I'm just really pleased that even though I may have been down in the game, I could hang in there and fight my way back," Stosur said. "If she was going to win the game she had to earn it from me. I wasn't going to give anything away today."
Cirstea is a 19-year-old high school student who had never before reached the third round of a major tournament. She may have been a little overwhelmed against Stosur, who broke the Romanian five times.
"I'm going to have to, once again, bring my good tennis to the court and play my own game," Stosur said of reaching the semifinals. "Doesn't really matter who's down at the other end at this point in time."
Stosur, seeded 30th, is the first Australian to reach the semifinals at Roland Garros since Nicole Bradtke in 1988.
"Just over the moon. Happy. Excited," said Stosur, who missed most of 2007 because of Lyme disease. "Every single positive emotion I think possible at the moment."
Information from The Associated Press was used in this report.