Spanish sharp-shooter Rafael Nadal overcame Frenchman Sebastien Grosjean and his noisily partisan crowd to remain on track to become the first man in 23 years to win the claycourt grand slam on his debut.
But Russian third seed Marat Safin went out, after a typically schizophrenic performance, 7-5, 1-6, 6-1, 4-6, 8-6 to 15th-seeded Spaniard Tommy Robredo.
Henin-Hardenne survived two match points to outlast U.S. Open champion Kuznetsova 7-6, 4-6, 7-5 and set up a quarter-final against Wimbledon champion Maria Sharapova.
"It's a kind of miracle," former world number one Henin said. "I managed to stay in the match even though I was not able to play my best tennis."
While the fancied Belgian 10th seed and 2003 champion survived, two Argentine favourites for the men's title were knocked out -- holder Gaston Gaudio and the man he beat for the 2004 title, Guillermo Coria.
Fifth seed Gaudio, who had not dropped a set on his way to the last 16, was beaten 2-6, 6-4, 7-6, 5-7, 6-4 by Spain's David Ferrer and eighth seed Coria was outhit by Russia's Nikolay Davydenko 2-6, 6-3, 7-6, 6-2.
SPANISH PRODIGY
Ferrer, who had trailed 4-0 in the fifth set, will meet compatriot and number four seed Nadal next after the brawny 18-year-old beat Grosjean 6-4, 3-6, 6-0, 6-3.
"I knew I could come back," Ferrer said. "In the end I just let my shots go and everything worked out.
"That's what tennis is all about. Anything can happen. Until the end of the match, you can't really say it's done."
The French crowd had halted play for nine minutes early in the second set of Nadal's match against Grosjean on Sunday, booing and jeering over a controversial line call.
The fans behaved after the match resumed on Monday and witnessed more great tennis from the Spanish prodigy who won the first three games to wrap up the third set.
The fourth was tight until Grosjean dropped serve by netting a backhand in the eighth game. Nadal served for the match and Grosjean hit a backhand wide on the second match point.
The Spaniard would not be drawn on his date with destiny. "Winning Roland Garros? Well, I'm in the quarter-finals now There are still eight. I'm one among eight players," he said.
Davydenko will play Robredo next after the Spaniard managed to hang in long enough for the volatile Safin to implode to record one of the best wins of his career.
"One of the best for sure, for the importance of the match, no?," he said. "Right now, I'm in the quarter-finals.
"This was a very tight match... I won by just one point... I was lucky. He got a little nervous. He's a player who tries to win every point. I think he committed more errors today than I did, and that was maybe the key to the match."
LIGHTER SIDE
Safin was sanguine in defeat.
"Not one thing went the way it could have gone," he shrugged. "I was always missing something. I can't say I was playing my best tennis. But I hope the crowd had a nice time... not much for them to complain about, eh?
"A lot of this is luck, you know? I have had a lot of luck. If I didn't have luck I would be washing bottles in Russia.
"There's not much I can say and I don't want to take anything away from him at all."
In contrast to Safin's failed efforts, compatriot Sharapova paid only a fleeting visit to Roland Garros on day eight of the claycourt grand slam.
She needed just three games to polish off Spain's Nuria Llagostera Vives 6-2, 6-3 in their rain-interrupted match held over from Sunday.
"I guess it was good just to have a good night's sleep," said the 18-year-old who is not afraid of Henin-Hardenne.
"I think I'm getting better and better. I've said this before, if people underestimate me on clay then they'll be surprised," she grinned.
"If the conditions are going to be heavier, a lot of the points are going to be long.
"I feel fit enough to be able to do that. It's just a matter of mentally going out there and doing it."
Sharapova's tactics spell bad news for Henin-Hardenne who has only recently returned after a knee injury and a virus that affected her badly last year.
"My body is suffering," said the Belgian. "I hope I can recover in time for my next match."
Men's ninth seed Guillermo Canas has no such concerns. The Argentine qualified for the quarter-finals without playing when his opponent, German Nicolas Kiefer, pulled out with a sore neck.
Kiefer, the 28th seed, was hampered by the injury in the fourth round when he took four sets to get past Russia's Igor Andreev. Canas will play Mariano Puerta next in an all-Argentine quarter-final.
Puerta beat compatriot Jose Acasuso 6-4, 6-1, 6-1.
