It was an indication of things to come - in the first set.
However, with nothing left to lose, Halep won eight straight points to move from 1-2 to 3-2.
Top-ranked Nadal enters Wednesday's match bidding to reach his 11th semifinal at Roland Garros.
However, Halep then broke Stephens' serve a third time to clinch the second set 6-4.
She reached two Grand Slam finals inside a year on the back of 10 months out of the game for foot surgery, and everyone is raving about the tennis Sloane Stephens is playing ahead of the French Open final today. It was overdue and well-earned.
"I think winning here is very special for her and I'm glad she finally got her first Slam".
Halep asserted her supremacy in the third set, almost handing Stephens a bagel and giving herself a sizable cushion for any late nerves. In the end, the match resulted in the superior player winning.
Between Stephens' finishing ability and Halep's overdue karma, something had to give.
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While Keys fired far more winners during the match - 25 to Stephens' eight - she also produced a whopping 39 unforced errors - 28 more than her opponent. Stephens, on the other hand, showed no signs of nerves in her one Grand Slam final win in NY previous year.
Halep looked bereft of ideas but finally started to find her range as Stephens' unforced error count rose quickly.
Stephens is an incredibly talented defensive player, and she kept stretching points until Keys would err. "He's a player that can be adapted more to the clay, it will be tough".
In fact, her next match win after the US Open did not come until Acapulco in February.
It was strikingly similar to what Halep did against Angelique Kerber in the quarterfinals.
"After not playing for 11 months, I've had some great results", said the American, who has a ideal 6-0 record in finals. "Then I changed a little bit of tactics and it worked".
Stephens handled defeat graciously. Muguruza essentially got that round off, too: Her opponent, Lesia Tsurenko, quit with an injury after only two games. You sense a confidence in her, an assuredness, which is hopeful. After sitting out almost a year because of surgery on her right foot in January 2017, she has soared up the rankings and now made it to her second major final in the past nine months. For sure he is beatable, but you have to have the level, and Dominic has the level and you have to play the ideal match and the most important thing is you can't miss the occasions that you have.
"I was dreaming of this moment since I was starting to play tennis". Halep chased down a drop shot and reached it by stretching into a semi-split; Stephens lofted the ball back, but Halep cranked a lethal backhand victor. They might be able to do this for another half-decade if both remain healthy. But she can become a captivating figure in American sports on her own terms.