In the other men's quarterfinal, No 3 Marin Cilic and No 5 Juan Martin del Potro were at 5-5 in a first-set tiebreaker when the matchup of past US Open champions was stopped.
Halep became the first woman from Romania to claim a Grand Slam title since her current manager, Virginia Ruzici, won the French Open 40 years ago.
But it was a different story entirely as the second set went on: Stephens slowed down and Halep turned up the heat with an intensity that led to her first Grand Slam title with a 3-6, 6-4, 6-1 win.
One thing Halep will not want to dwell on is the fact she has won only one of her past eight finals - including a defeat by Jelena Ostapenko in last year's Roland Garros showpiece.
She first came close to winning the French Open in 2014, losing a three-set final to Maria Sharapova.
A foot injury - and then surgery - had put her career on ice for 10 months and her ranking had nosedived to 323 in the world.
The psychology of this final involved Halep desperately wanting to break her duck at the Majors and Stephens having the security of already knowing that she had one in the bank. And when I broke her first game, I felt more confident and I felt I have the game in my hands.
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.
Nadal lost the opener 6-4 against 11th-seeded Diego Schwartzman but began to play better after a rain delay of just under an hour. "I don't have words to explain what this means to me and my team". So I have nothing to lose tomorrow. Obviously, I grew up in the States where we don't really have red clay.
- Juan Martin del Potro nearly didn't play in Paris due to injury, but will take on Rafael Nadal in the last four on Friday. "You have to take it". But nothing is impossible.
"I think Serena hated me for being the skinny kid who beat her, against all odds, at Wimbledon", Sharapova wrote in the book. "So to be here from there, I think, is rewarding, because those times were not easy".
"It was someone who made a loud (noise) before my serve, and I made double fault. I don't get too up, too down", said Stephens.
"After the first set I just stayed strong, I did not give up at all", said Halep. "But I'm still trying to find the right person".
"It's tough to speak now", said an emotional Del Potro. "I'll come here Friday, have a hit, have lunch here, go to my room, watch TV, wait till dinner, have dinner, go to sleep". I think that's a load of crap, but, you know, it's just me (smiling).
She elected to receive and looked nervous from the start as Stephens easily held in the opening game. "I think everyone would ask me questions, as well".
The United States trade representative office told the Washington Post that the USA actually has a trade surplus with Canada. You know, whether you like it or not, and it may not be politically correct, but we have a world to run.
Just 0.85 percent responded that they had previously wanted Northern Ireland to join Ireland and now wanted to remain British. Both sides want a deal by October to avoid barriers on the Irish land border that might fuel new conflict in the north.
Liverpool were close to signing Shaqiri from Bayern in the summer of 2014 and were the player's preferred option at the time . He continued: "We need three or four players in, but if we get that, I think we can genuinely challenge Manchester City ".
The news comes alongside the reveal of Origin Access Premier, which will give access to even more games in the EA online library. That has been a constant complaint by gamers, in fact, and something Electronic Arts has made a decision to address today.
Zampella said that it would be during the "dark times" and that naturally, players will pick up a lightsaber during the game. EA Games, which acquired Respawn in 2017, confirmed that the game centers on "a surviving Padawan".
Morgan chief executive Jamie Dimon appeared on CNBC on Thursday to discuss why he doesn't think tariffs are the way to go. Dimon has blasted excessive reporting requirements and the short-term focus of quarterly earnings.
In appearances before congressional panels since then, Pruitt has repeatedly blamed subordinates for his alleged ethical lapses. Greenwalt worked in several roles at the Oklahoma attorney general's office under Pruitt, lastly as his general counsel.
The Duke of Edinburgh, who celebrates his 97th birthday on Sunday, has retired from official public duties and did not attend. Also riding in the ceremony for the first time was the Duke of York in his new role as Colonel of the Grenadier Guards.