A nervous start from Muguruza was immediately pounced upon by Halep for a break in the opening game.
Halep's third final at Roland Garros comes 40 years after the first and only Romanian triumph in a women's final at the French Open. Halep started slowly, spraying shots wide as she lost the first four games of the match against the No. 12 seed who hit her early groundstrokes with lethal accuracy.
"I have just to stay strong, to try to make her uncomfortable on court and to try to play my game", Halep said.
In Thursday's other semi-final, US Open champion Stephens will take on fellow American and close friend Keys who she beat 6-3, 6-0 in the final in NY last September. In the victory, Halep won 61 percent of her first serve points and 53 percent of her second serve points.
He briefly lost his focus toward the end of the second set when he appeared to be distracted by a spectator just before hitting a double-fault to drop his serve and give Cilic a 5-4 lead.
Two-time major victor Kerber raced into a 4-0 lead in the first set before having to rely on a tiebreak to nudge her ahead.
The friends hugged at the net, both smiling.
Halep's fourth chance to win her first Grand Slam title will come against reigning U.S. Open champion Stephens on Saturday.
Halep promises to offer a more stern challenge, having won five of her seven meetings with Stephens, but the American will hope the world No. 1 will again falter in the final of a Slam, a stage on which she has so far failed to produce her best.
Halep assured herself of retaining the No. 1 ranking and reached her third final in Paris.
"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", Sharapova said.
"I think Serena hated me for being the skinny kid who beat her, against all odds, at Wimbledon", Sharapova wrote in the book. The last time the two were scheduled to play was in Qatar in February, but Halep was forced to retire. I think I have to put a lot of things in perspective for the last, what, four, five weeks since the clay season.
The win marks del Potro's first appearance in the French Open semifinals since 2009. Now I just have to go find her, because I need to tell her some juicy stuff. Three previous times, the other player - be it Maria Sharapova, Jelena Ostapenko or Caroline Wozniacki - won that third set. "I think my body will need a little bit of rest, but mentally I'm ready to go", said five-time major victor Sharapova. "So I didn't really know what to do". However, she stormed back to win the next two sets 6-3, 6-2 and get the win. She's clear on who she is as a player.
Sharapova looked nervous from the beginning and was never allowed to settle as Muguruza posted her first victory against the Russian at the fourth attempt.