Halep went all out in the first set but her bold strategy backfired badly as she hit 30 unforced errors.
A win at the Miami Open in April seemed to turn things around, but she followed that by failing to advance past the third round in each of her next four tournaments.
A first hold and subsequent break in the second set helped the Spaniard to 4-2 before the Romanian responded to level at four apiece. "If I do that, I will be OK after the match, no matter the result".
"I have actually been told quite often that I'll never win or do well because I'm too nice of a person and I just don't have it. It's been a little bit longer for me to get used to it, but I feel like every year I get more comfortable".
Halep's win knocked Caroline Wozniacki out of the race to be number one in next week's rankings, and set up a winner-takes-all match against Spain's Muguruza in Thursday's semi-final. She lost both of the previous ones in three sets, to Maria Sharapova in 2014 and to Jelena Ostapenko in 2017.
In addition to Halep v Muguruza, the other women's semifinal is the first at the French Open featuring two Americans in 16 years: No 10 Sloane Stephens against No 13 Madison Keys in a rematch of last year's US Open final.
And it was Sharapova who looked the rustier, with two limp service games handing Muguruza an immediate 4-0 lead and consequently the first set.
A marathon ninth game proved pivotal as Halep saved three break points to pile the pressure on the third seed. Halep has won 12 of her last 14 matches on clay court.
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A sweeping, running forehand into an open court gave her the set with Muguruza having managed just two winners. For good measure, remember, Stephens threw in a rut between the U.S. Open last September and her one-round Australian Open stay last January, but as everyone aimed to decipher that, and Stephens aimed to refrain from panic because that's who she is, Murray offers a plausible explanation.
She needed only 70 minutes to eliminate Sharapova and dominated her with superb returning, breaking the former No. 1 a half-dozen times. She double-faulted three times in the opening game, part of her 27 unforced errors. Muguruza essentially got that round off, too: Her opponent, Lesia Tsurenko, quit with an injury after only two games.
Muguruza will face Simona Halep or Angelique Kerber in the semifinals.
It was the first set that the Wimbledon champion had dropped at Roland Garros this year.
History is on Nadal's side as he continues his quest for an 11th title at Roland-Garros.
The 10th-seeded Stephens had never made it past the fourth round in Paris until now.
In the other men's quarterfinal, No 3 Marin Cilic and No 5 Juan Martin del Potro were at 5-5 in a first-set tiebreaker when the matchup of past US Open champions was stopped.
Maria Sharapova left the French Open without much of a fight, but had enough left in the tank to deliver a parting shot at Serena Williams. "Like I said, you have to just take your opportunities when you get them, and it's just match by match, basically".
"She's No. 1 in the world for a reason", Stephens said of Halep. "It is a very tough day, because, you know, it's one of those matches where you train so hard, you know, to achieve a Grand Slam final".