Santangelo, ranked No. 129, held serve to force the tiebreaker. She fell behind 4-1, but rallied to tie it at 4 with a volley that had her skipping with delight back to the baseline.
But Henin-Hardenne regained control, ending the match with a forehand pass that just clipped the line. She finished with 24 winners and 29 unforced errors.
"It wasn't an easy match," she said. "I won the first set pretty easily, with a lot of intensity, then, at 1-all in the second, the intensity came down a little and she just took her chances -- it was a big fight in the second set."
Henin-Hardenne beat Davenport in the fourth round last year, rallying from a 4-1 deficit in the deciding set and overcoming cramps to win 7-5, 5-7, 9-7.
Mauresmo struggled to beat the 40th-ranked Molik after losing only six games in the first three rounds.
"Obviously she gave me a lot of trouble," Mauresmo said. "I just really wanted to go through this one and go to the quarters."
Mauresmo, a finalist in 1999, missed the tournament last year because of injuries and said she was bothered by back trouble against Molik, but didn't think it would upset her campaign.
"I'm going to go for treatment and make sure everything is OK for the quarterfinals," she said.
With the victory, Mauresmo is assured of breaking into the top three for the first time when the next WTA Tour rankings are released. Her career high was No. 4 in October 2002.
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