Keys briefly rallied to pull one break back, but Stephens did not waste her second opportunity, sealing victory on her first match point with a glorious backhand victor down the line.
She then managed to fight off incredible resistance for a hold that saw Muguruza miss three break point chances and held off the challenge to advance to a third French Open final.
While the crowd are likely to get behind the world No 1 Halep, Stephens' nerveless approach to her game could prove a tough obstacle to clear. 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. I don't think you have to be mean in order to win matches.
Predictions: who will win the women's final? Of her first 14 points, 13 arrived via errors by Muguruza - nine unforced, four forced.More news: Ruthless Rafael Nadal crushes Del Potro to reach final
She needed only 70 minutes to eliminate Sharapova and dominated her with superb returning, breaking the former No. 1 a half-dozen times. I tried to do too much.
With memories of their epic Australian Open semi-final in January, which Halep edged 9-7 in the third, still fresh in the minds they went to a decider. I will play for the fans from all over the world, because I know that many are hoping me to win this Grand Slam finally.
The No. 3-seeded Muguruza had lost all three previous meetings against Sharapova, who won two of her five Grand Slam titles at Roland Garros.
It was 5-0 by the time Muguruza eventually claimed a game. Stephens has now followed that up by once again defeating Keys to reach the final in Paris. She had 41 unforced errors today, which isn't going to win you too many two-set matches.
But Keys, who made 23 unforced errors in the first set, hit back hard and set up a break point at 3-2, only for Stephens to save it with a service return victor. "I think Serena hated me for being the skinny kid who beat her, against all odds, at Wimbledon", Sharapova wrote.More news: Zinedine Zidane's Successor At Real Madrid Revealed?
"There were a couple matches where she was probably like, "Oh, my God, ' right?" said Stephens" coach, Kamau Murray.
Halep had another 24 errors against Garbine Muguruza in semifinals, which while not ideal, is still a pretty clean match for someone with Halep's margin for error.
Like Halep, Stephens is an incredibly talented defensive player, and she kept stretching points Thursday until Keys would err. She really deals with things better. Some of those matches were very defining for me.
On Thursday, it was more of the same as the 25-year-old capitalised on her friend's big match nerves. I will say this, though. "I think that we'll just have to go out and compete". It would be very unusual, I think, if I didn't write anything about her.
On Saturday, the pressure will definitely be on Halep - she is the top seed, she has been here twice before, she is playing on her favourite surface and she leads the head-to-head.More news: Trump says he's considering pardon for Muhammad Ali
But the American 10th seed has never lost a WTA final, with her triumph in the Miami Open in March her sixth win from six.
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