The 10th-seeded Belgian made 58 unforced errors but fought back strongly to extend her winning streak to 18 matches and set up a clash with U.S. Open champion Svetlana Kuznetsova.
Henin-Hardenne won consecutive titles in Charleston, Warsaw and Berlin in the run-up to Roland Garros, but said the win over Garrigues was all the more satisfying because of the effort of their two hour, 17 minute clash.
"This was a real clay tennis match," Henin-Hardenne said.
"We don't see many on the female tour and I think this has been one of my best matches. She didn't give away anything. I had to do it all. It really wasn't easy."
Garrigues, the world number 34, frustrated Henin-Hardenne with some outstanding retrieving -- with one behind-the-back shot the highlight -- as she took a 5-0 lead.
Henin-Hardenne hit back to 5-4 only to drop her serve and lose the set, and it took a huge effort for her to eventually grind Garrigues down.
"I don't think I played really badly at the beginning," she said, despite taking 25 minutes to win her first game.
"I think she was playing good and I was a little bit surprised by the way she was playing. She was really consistent and she wasn't making mistakes.
"I think it was a very good match, very high level. She pushed me a lot. This type of match is very satisfying because you really need to go for it. That's what I tried to do. I was never discouraged and it's very positive."
Henin-Hardenne will have little time to rest before she faces Kuznetsova on Sunday. She has won six of her seven previous meetings with the Russian, though the last, in Warsaw earlier this month, was a tight three-set battle.
"Kuznetsova is a very tough opponent, particularly on clay," she said. "She's extremely powerful. She can be quite dangerous.
"I beat her in Warsaw in a very long match. That was quite a fight. Tomorrow is going to be a new battle, and we shall see."
