"I rushed too much at the beginning", Halep told a news conference. In the last game I felt I could not breathe any more.
"She was very strong on court and when I was down a break in the second set I said, "Okay, everything is gone, I have just start [sic] to relax and to enjoy the match ... and I came back".
U.S. Open champion Stephens used her powerful game to full effect to take the first set but then ran out of steam as Romania's Halep completed a memorable victory.
By the end of the match the unforced errors kept creeping into Stephens' game and at times she looked totally lost as she tried to work out what was happening.
"I said there is a chance to come back and win it".
There was also a big hug for coach Darren Cahill who was in the losers' box a year ago when Halep blew a commanding lead against Jelena Ostapenko in the final. "But in tennis, you never know, so I will stay chill".
Stephens wrapped up the set with a love hold and broke in the first game of the second as the errors continued to rack up for Keys, who surrendered her serve with a volley into the net.
"This is not the trophy I wanted but it's still lovely", Stevens said after collecting the silverware as the runner-up. She's won 7.6% more points on serve (holding a magnificent 87.3% of the time) but Halep has won 3.7% more points on return (breaking opponents a stratospheric 63.3% - the best return data I can remember seeing for a long time).More news: How to watch online unveiling of new BlackBerry phone
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Halep entered the final as the favorite given her clay-court prowess, higher ranking and holding a 5-2 head-to-head record against her rival.
The world number one, who had a reputation for collapsing when it mattered after losing three Grand Slam finals including two in Paris, finally put her hands on a major title after showing great determination to tame American Sloane Stephens 3-6 6-4 6-1 in the title match at Roland Garros.
She persevered through a battle to hold serve and go up 5-4. Halep's stamina and will to rally from being down a set is something that sets her apart from numerous best players in the world. The final string snapped in the third set with Stephens trying to stave off a break point at 0-3.
"The grass is coming, I have few tournaments ahead, but now I'm off", she said.
"So we will see what is going to happen, but I just want to be smiling on court".
As Halep scurried to chase a shot that turned into a drop shot for Stephens and won the point for 4-0, her coach Darren Cahill leaped out of his chair and pumped his fist. But in the end, she couldn't quite overcome Halep's excellence.
A first-ever Grand Slam final defeat in straight sets for Halep looked likely as soon as Stephens claimed the first game of the second set on her fourth break point.
'Congratulations on your first Slam. The semi-final will have added stakes as the victor will become the women's world number one.