Halep took her place in a second consecutive Grand Slam final after losing in the final of the Australian Open in January to Caroline Wozniacki and fourth final at the majors overall.
Former number one Muguruza takes on current incumbent Halep on Thursday with the victor to be assured of the top ranking as well as a place in the Roland Garros final.
Muguruza ratcheted up the intensity in the second set and broke in the third game when a powerful return forced an error, only for Halep to fight her way back to level at 4-4. Halep will also have to contend with the fact that once she makes it to the second week of slams, Muguruza comes alive.
All three were tight, three-set defeats, none more painful than her slow fade against the inexperienced Ostapenko in Paris a year ago, where she led, 6-4, 3-0, with a point to go up, 4-0, before Ostapenko rallied to take the trophy instead.
Muguruza is 3-0 in career grand slam semifinal matches.
Following her 70-minute defeat to Garbine Muguruza 6-2, 6-1 in the quarterfinals at Roland Garros on Wednesday, the Russian spent a portion of her press conference addressing criticism made by her rival about her autobiography, in which Williams claimed the book was "100 percent hearsay". She was prior to the draw and it took a gift draw for Elina Svitolina - which the Ukrainian was unable to exploit - for her to lose this status for the opening two rounds. She's a great opponent, a great player.
The victor of Thursday's semi-final will also decide the world number one spot which Muguruza held for four weeks past year.
"I'm happy that I can have another chance". It was yet another example of just how commanding the 24-year-old former French Open champion can be when she's keeping self-doubt at bay. "I would just call it a road".
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The World cup is not just about winning the cup; it is not just about participation, it is also about making and breaking records. Since then, Russia's team have shown little progress.
An anti-climactic match ended predictably, with a Sharapova backhand nestling in the net - her 27th unforced error.
"To have had the victories that I have had, to have the results that I have, obviously moving a step in the right direction", said Sharapova who had made the last-eight in Madrid and semis in Rome in the run-up to Paris.
Keys will have to be patient in this semi-final, and only pull the trigger when she's ready to finish off the point.
"Then I have two more days until I play the final".
Muguruza eventually got on the board and started to settle into the second set, breaking serve and getting to within two games of a decider at 4-2.
But I also think Stephens has figured out the awkward dynamic of playing your good friend better than Keys has. I think that's a load of crap, but, you know, it's just me (smiling).
But by the third set Halep had broken the German's defenses - and her resolve - as she banged in 76 percent of her first serves, with only half the unforced errors of her opponent.
But the dogged Romanian scrapped hard to turn the contest on its head to win 6-7 (2-7) 6-3 6-2 to advance to the final four.