However, Halep is a two-time runner-up at Roland Garros - slumping to a defeat to Russian tennis star Maria Sharapova in the 2014 final before losing to Jelena Ostapenko a year ago.
Top seed Simona Halep fought back to triumph in a bruising French Open quarter-final battle against Germany's Angelique Kerber and stand two wins away from her first Grand Slam title.
US Open champion Stephens had looked set to lift a second Grand Slam trophy when leading 6-3, 2-0, but the American 10th seed wilted in hot conditions and Halep pounced. "I don't want to think about that.it's a different match, different situation", she said.
"I feel calm. I feel that I have to enjoy it today, the victory, because it was a great match".
The world number one had lost all three of her previous major finals, including two at Roland Garros, and said she struggled to contain her emotions after staging a thrilling comeback. So I will stay chilled.
She improved in the second set, breaking early on only to be pegged back at 4-4 before Halep held a marathon ninth game and broke for a 6-1 6-4 win.
Dominic Thiem beats Marco Cecchinato, books French Open final birth
As for what sort of tactics will be employed Sunday, Nadal's coach, Carlos Moya , offered a hint. Of that set, Thiem said: "If I had lost it, it would have been a very close match".
The match was all but ended as Halep clinched the double break in game four after some magnificent defending in one of the points of the match.
Sloane Stephens maintained her ideal record against fellow American Madison Keys as she blazed into the French Open final with a 6-4 6-4 win on Thursday in a repeat of last year's U.S. Open final.
"It's always hard to play someone from your country and such a good friend, but I'm happy I played my tennis", Stephens said.
This was a less clear-cut victory but an early break of serve in each set put Stephens in control.
"This is one of my favourite tournaments and I'm looking forward to Saturday".