The 25-year-old's ranking slumped to 957th after an 11-month absence and a first-round defeat at Wimbledon last year, but she bounced back in stunning fashion by winning the US Open against her close friend Madison Keys - the same player Stephens beat in the semi-finals at Roland Garros on Thursday.
Stephens started the game aggressively but Halep was able to hold her own to start with. The last Romanian woman to have won the Paris Open, in 1978, now works as an agent for Halep and had been cheering her from the stands. Halep, one of eight women in the Open Era to lose in their first three Grand Slam finals, must dig deep to win every point given Stephens hasn't much court to cover behind the baseline.
Muguruza pounced on Sharapova's errors to lead 4-0.
Halep ended the match when Stephens returned a serve into the net.
She broke in the fourth and served out for the set in the ninth game, showing some impressive defence against an increasingly aggressive Halep.
However, Halep then broke Stephens' serve a third time to clinch the second set 6-4.
"I don't know if I will win, but I will take a big love from you and that's the most important for me", he said. "I should've changed my game when I noticed she was changing hers".
"I think she's had a tough journey".
Halep went all out in the first set but her bold strategy backfired badly as she hit 30 unforced errors. Stephens has one grand slam title under her belt, which came in the 2017 US Open.
She'll reach the top five anyway next week, but what happened just after the 2-0 lead wound up ratifying the idea of Halep as a fighter, an idea cemented when she lost the 2018 Australian Open final to Caroline Wozniacki in three sets and wound up treated for dehydration. The semi-final will have added stakes as the victor will become the women's world number one.
Many Americans struggle on the red dirt because of the lack of red clay courts in the United States but several players have managed to master the surface nonetheless. Halep does not possess the power of Keys, and Stephens has shown she is one of the best movers in the game.
The 26-year-old had to wait for her 32nd appearance at a Grand Slam event to win one at last.
"No matter how hard the adversity that you go through, there is always light at the end of the tunnel, and I'm glad she finally got her light". She made the last-16 in Madrid and Rome and now has a chance to make the French Open final.
Stephens will give it her best shot but Halep is the favorite for a reason.