It's the first time either player has made the last-four in Paris and the two friends are looking forward to their third career showdown against one another.
The American pair have contrasting styles, with Stephens' game based on her athleticism and ability to get many balls back, while also turning defence into offence fairly quickly.
"I started in the first set not matching her level", said Muguruza, the reigning Wimbledon champion.
"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". I didn't, like, try to do it. "Then I changed a little bit of tactics and it worked". I'm happy to play the final again at my favourite grand slam.
Halep had to come back from a break of serve down in the second set and fend off three break points at 4-4 before finally closing out the victory, which guaranteed that she will remain No. 1 in the rankings after this tournament.
After a timid start, Stephens found her rhythm and took control of the opening set by breaking in the third game with a forehand victor down the line.
Halep assured herself of retaining the No. 1 ranking and reached her third final in Paris.
Muguruza had thrashed Maria Sharapova in the quarter-finals but Halep was a different proposition entirely, and it seemed she knew it. She committed four double faults, compared to one for Muguruza, and had just one ace.
Keys made it to the semis with a straight-sets win over Kazakhstan's Yulia Putintseva while reigning US Open champion Stephens eased past Russia's Daria Kasatkina.
She needed only 70 minutes to eliminate Sharapova and dominated her with superb returning, breaking the former No. 1 a half-dozen times.
Having saved three break points to consolidate her lead, Halep continued her onslaught on Muguruza's shaky serve. He committed 74 unforced errors on Thursday, one more than in his previous two matches combined. Muguruza essentially got that round off, too: Her opponent, Lesia Tsurenko, quit with an injury after only two games.
Halep got off to a poor start, with 10 unforced errors in the opening three games handing Kerber a double break.
History is on Nadal's side as he continues his quest for an 11th title at Roland-Garros.
Two past U.S. Open champions meet in the other men's quarterfinal: No. 3 Marin Cilic against No. 5 Juan Martin del Potro.
"Coming into this part of the year, I was losing a few first-round matches, matches that I wanted to be winning, of course", Sharapova said. Halep, who has yet to win a major championship, is in the final for the second consecutive year.