The Romanian underlined that pedigree with a straight-sets scorching of Spaniard Muguruza-who is herself a daunting figure on clay.
"After the first set I just stayed strong, I did not give up at all", said Halep, who is still chasing a maiden grand slam.
"If I'm able to play my game every match I play, then I have a better chance to win. My head won it".
Simona Halep is two wins away from earning her first Grand Slam title.
Like Halep, Stephens is an incredibly talented defensive player, and she kept stretching points against a mistake-prone Keys, who made 41 unforced errors - 30 more than Stephens.
Halep had started slowly in her quarterfinal match against Angelique Kerber on Wednesday, though the top seed did not have a repeat of that today, dominating the first set against a Muguruza who was simply overawed by the tennis against her. She also was coming off a lopsided victory in the quarterfinals a day earlier, overwhelming five-time major champion Maria Sharapova 6-2, 6-1. "So to be here from there, I think, is rewarding, because those times were not easy".
Sharapova certainly didn't do that.
They have won three Grand Slam titles together: the French Open in 2013, the U.S. Open in 2014 and Wimbledon a year ago.
It was 5-0 by the time Muguruza eventually claimed a game. "Like I said, you have to just take your opportunities when you get them, and it's just match by match, basically".
And it was Sharapova who looked the rustier, with two limp service games handing Muguruza an immediate 4-0 lead and consequently the first set.
Sharapova didn't play a point in the fourth round, because Serena Williams pulled out of their much-anticipated showdown with an injured chest muscle.
Her craft and guile pushed Sharapova all the way four years ago but couldn't get her over the finishing line, while it was Latvian Ostapenko's ferocious hitting that produced the stunning comeback in 2017.
When asked June 3 about the remarks ahead of bowing out of the French Open because of an injury, Williams said she was surprised that so much of the book was about her.
She improved to 3-0 against her longtime pal and Fed Cup teammate Keys, including their U.S. Open final last September.
Stephens's run to Saturday's final comes after a roller-coaster year since her US Open stunner.
Halep, meanwhile, has participated in a trio of major finals, losing each. But I remember, like, the first week of French Open, I was at a wedding in Ireland. That set the stage for the lengthy ninth game where Halep stood firm.
But Halep figured things out. She's suffocating her opponents with such deep shots and bullying her way across the court.
She admitted afterwards that she felt sick with nerves, adding "maybe I wasn't ready to win it, but maybe next time". A lot of emotional stuff: am I ever going to be the same, am I ever going to play good again at a high enough level, am I a protected ranking? Some of those matches were very defining for me.