Keeping Williams on the move with versatile shotmaking, two-time defending champion Henin advanced to the semifinals by winning 6-4, 6-3 Tuesday.
Henin extended her Open era record winning streak at Roland Garros to 31 consecutive sets. The Belgian is two rounds from becoming the first woman to win three consecutive French Open titles since Monica Seles in 1990-92.
Williams, who had won her past 11 Grand Slam matches, was the lone remaining American in men's or women's singles. She was puzzled by her poor play and reluctant to credit Henin.
"All she had to do was show up," Williams said. "I just pretty much stood back and let her take advantage of me."
Responded Henin: "It's her opinion. I thought I just did a good job. And I just see it from my point of view, and I did everything I could to control the match."
Justine Henin's aggressive play was evident as she managed to 'impose her game.' (AP)
Top-ranked Roger Federer set an Open era record by extending his winning streak in Grand Slam matches to 36 consecutive sets -- then lost the next set. He regrouped and hit a series of brilliant shots down the stretch to beat No. 9-seeded Tommy Robredo in the quarterfinals, 7-5, 1-6, 6-1, 6-2.
"I was struggling, it's true," Federer said. "I was like, 'OK, this could start becoming really bad here.' Thank God I played a great third and fourth set."
In the final two sets, he had 26 winners to five for Robredo.
Federer, who has reached the semifinals in a record 12 consecutive major events, is seeking the only Grand Slam title he has yet to win. He'll next play No. 4-seeded Nikolay Davydenko, who beat No. 19 Guillermo Canas 7-5, 6-4, 6-4.
Federer is 8-0 against Davydenko, including a semifinal win at the U.S. Open last year.
The other women's semifinal will be between No. 2 Maria Sharapova and No. 7 Ana Ivanovic. Sharapova beat No. 9 Anna Chakvetadze 6-3, 6-4, and Ivanovic defeated 2006 runner-up Svetlana Kuznetsova 6-0, 3-6, 6-1.