But she found form with back-to-back semi-finals in Toronto and Cincinnati before winning the US Open as the world number 82. Halep sealed her victory with 6-4 in the second set.
She further proved her credentials with a dominant, yet gutsy, win over 2016 champion Muguruza, who failed to hold serve in the first set.
Halep has won her last four matches against Stephens and both of their meetings on clay, including in the 2014 French Open fourth round, while the two have never played a three-set encounter. But the Romanian clawed her way back into the contest, winning three straight games as both players struggled to hold on to their serves in a monster opening set that swung one way and then the next.
Now she has another chance to break her duck when she takes on American 10th seed Sloane Stephens in the final at Roland Garros.
Her reward is a showdown with American Stephens, who maintained her ideal record against compatriot and friend Madison Keys, as she blazed into the final with a 6-4 6-4 win in a repeat of last year's US Open final.
After simply being swept aside in the US Open final back in September, Keys was looking much stronger here, with the younger of the two Americans looking confident when serving despite that wobble in the third game.
Simona Halep may have finally learnt to exhale in a Grand Slam.
"I started in the first set not matching her level", said Muguruza, the reigning Wimbledon champion.
"She's No. 1 in the world for a reason", Stephens said of Halep.
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It's an honor and a privilege to work for the president and to be part of his administration. "And they should be fired". The president, who very rarely says sorry, did not demand an apology - even though other aides said Sadler should.
She admitted afterwards that she felt sick with nerves, adding "maybe I wasn't ready to win it, but maybe next time". The 25-year-old has proven herself in those situations, and Halep will need to come up with her most daring counter yet.
The world number one trailed 6-4, 3-5 when their quarter-final was halted due to rain yesterday, but recovered to win the next three sets 6-3, 6-2, 6-2.
Top-ranked Nadal enters Wednesday's match bidding to reach his 11th semifinal at Roland Garros.
Sloane Stephens of the United States returns the ball to Madison Keys the United States during their women´s singles semi-final match on day twelve of The Roland Garros 2018.
Halep is more emotive on court and her tenacity has earned her an army of fans.
"I think the book was 100 percent hearsay, at least all the stuff I read and the quotes that I read, which was a little bit disappointing", said Williams, who played in her first Grand Slam since giving birth past year.
But after a solid claycourt season in which she reached the quarter-finals in Madrid and semis in Rome, Sharapova appears to be heading in the right direction and she will be close to the top 20 again when the rankings are published on Monday.
He will now face another Argentine, Juan Martin del Potro, in Friday's semifinal, after Del Potro conquered Marin Cilic 7-6 (7-5), 5-7, 6-3, 7-5.
Kerber is a two-time Grand Slam champion. She's suffocating her opponents with such deep shots and bullying her way across the court. "If I do that, I will be okay after the match, no matter the result".