Stephens was smooth and resourceful, mixing booming topspin with flatter, faster strikes in a way that made her - for 40 minutes - unbeatable.
The apparent rout continued into the early stages of the second set, as Stephens broke again to lead 6-3, 2-0. The scoreline then was curiously similar to the lead Halep held over Jelena Ostapenko in Paris last year.
Halep wound up as the frustrated party that day, backing off at just the wrong moment and allowing the free-swinging Ostapenko to surge to a shock win. But she is never more dangerous than when trailing, because that's when she loosens up and starts playing like a woman with nothing to lose.
"I thought, 'Everything is gone. I just have to relax'," said Halep, grinning her way through her on-court interview.
The switch was partly spiritual, as she tapped into her deepest wells of tenacity. But it was also tactical, as she decided to use all the court for the first time, darting to the net whenever Stephens was pushed deep.
Commentating for ITV, former Wimbledon champion Marion Bartoli began yelling: "That's the play! That's the play!" whenever Halep put away a winning volley. There is a lot of warmth towards Halep in tennis, as she always conducts herself with class. If Halep found her best self in her moment of crisis, Stephens began to show signs of weariness. The match was only an hour old but Halep was setting a demanding pace and giving fewer and fewer freebies away.
No fewer than 55 rallies exceeded nine shots, more than a third of the total points. The average speed of Stephens' forehand dropped from 124km/h in the first set to 112km/h in the second.
The third set might look like a non-event on the scorecard but some of the rallies were extraordinary. And none more so than the point Halep won to go a double break ahead at 4-0. This was a 16-shot extravaganza in which she had to dig out a couple of desperate retrievals from the back of the court, then sprint up to the net to dink back a drop-shot, and finally backpedal to make that most awkward of putaways, the backhand overhead. When she finally edged Stephens out in that one, the usually sphinx-like Cahill jumped up off his seat and punched the air.
"I felt your support," Halep told the crowd after the match. "I have been dreaming of this moment since I started to play tennis. I'm really happy that it's happened in Roland Garros in Paris, my special city."
Martina Navratilova came up with a neat summation: "It wasn't a monkey off her back; this was an 800lb gorilla."
Stephens did not win too many friends in the interview room afterwards when she complained about the emphasis placed on her record outside North America (no wins and nine defeats this year, before the French Open). "You guys are my biggest haters," she said.
Stephens also groused about the size of her cheque, saying "I heard what it was [around $1.9 million] and I'm a little let down."
This was the seventh straight women's grand slam with a different winner, a sequence that began with Angelique Kerber in New York two years ago through to Serena Williams, Ostapenko, Garbine Muguruza, Stephens, Caroline Wozniacki and Halep.
