Halep started slowly, spraying shots wide as she lost the first four games of the match against the number 12 seed who hit her early groundstrokes with lethal accuracy.
And she admitted that even in her quarter-final clash with Kerber on Wednesday, it was her mental game that finally gave her the edge.
There were no such fireworks this time, though, with Halep sealing a 6-7 (7-2) 6-3 6-2 victory.
She needed only 70 minutes to eliminate Sharapova and dominated her with superb returning, breaking the former No. 1 a half-dozen times. "My head won it", Halep said.
Spain's Garbine Muguruza has yet to drop a set in the tournament. The victor earns a berth in the final at Roland Garros, plus the number one ranking that now belongs to Halep.
"She did a lot of things better than I did", Sharapova said in a press conference after the match. "She loves clay. She loves Roland Garros".
During the changeover after the set, a trainer came out to tape both of left-handed Nadal's wrists.
Two past U.S. Open champions meet in the other men's quarterfinal: No. 3 Marin Cilic against No. 5 Juan Martin del Potro. Stephens won that one, as well as the only other match the longtime friends, Fed Cup and Olympic teammates have played against each other.
Muguruza had lost all three previous times she faced Sharapova, whose five Grand Slam titles include two at Roland Garros.
Angelique Kerber appeared on her way toward an easy win like Muguruza, going up 4-0 in the first set.
Sharapova certainly didn't do that.More news: Here are the highlights from Apple's 2018 Worldwide Developers' Conference
Serena Williams might have fancied meeting this version of Sharapova, who made six double faults and 27 unforced errors. She reacted well to limit her mistakes while staying aggressive on her German opponent's service games.
"I think there is a lot of autobiographies out in the world, especially in the sporting world, that don't necessarily speak about whether they were rivals or someone they competed against".
"Kind of a weird day", Muguruza said.
The 24-year-old Garcia's face was in the front page of L'Equipe on Monday, quoting her as saying her goal was to "win a Grand Slam", and she did not change her statement after her 6-2 6-3 defeat on Court Suzanne Lenglen.
She could be on track for another trophy after winning this event in 2016.
The last French woman to win a singles Grand Slam title was Marion Bartoli, when she won Wimbledon five years ago.
Halep, meanwhile, has participated in a trio of major finals, losing each.
Follow along for a breakdown of quarterfinal action at the second Grand Slam of the year.
Halep broke Kerber in the very first game and then managed a strong hold to go 2-0 up.
But Halep figured things out. If she can channel that same degree of aplomb and expertise in the semi, Halep doesn't have much of a chance.
By the time Sharapova recovered somewhat to play better it was too late, and Muguruza was into her impressive stride, with the court finally filling up post-lunch.
"No expectations, no pressure", Halep said as she looked ahead to the semifinals, perhaps trying to persuade herself as much as anyone.