Halep does not possess the power of Keys, and Stephens has shown she is one of the best movers in the game.
Daniel Rapaport: In a final featuring two players as evenly matched as Halep and Stephens-both will be top-five after this tournament, and both are without question one of the five best players in the world-the decisive factor is often mentality.
On Wednesday, Halep looked anything but the top seed, slipping 4-0 down in the first set before she rallied to defeat Kerber, a two-time major victor who had exited in the first round in Paris in the last two years. I tried to do too much.
"It's always a tough match when I play against her".
"To me, it is really going to depend on who's the most fearless and who's going to come out the box and play more aggressive tennis".
This will be the world No. 1's fourth major final and her second trip to the last stage at Roland-Garros in as many years, having suffered a shock defeat at the hands of then-teenager Jelena Ostapenko in 2017.
Muguruza pounced on Sharapova's errors to lead 4-0.
She improved in the second set, breaking early on only to be pegged back at 4-4 before Halep held a marathon ninth game and broke for a 6-1 6-4 win.
"Before, I gave it a lot of importance so that's why you're fighting every week, and I got there".
"She's No. 1 in the world for a reason", Stephens said of Halep.
"I will play for the fans from all over the world, because I know that many are hoping for me to win this Grand Slam finally", vowed Halep.
But sport is often a cruel and cold mistress, and in Sloane Stephens Halep faces another serious test of her nerve.
- Robin Soderling, one of only two men to beat Rafael Nadal at Roland Garros, thinks the Spaniard can win the title for an 11th time. So for me staying calm is the most important thing.
Sharapova, 31, was broken in the opening game of the second set which she immediately retrieved.
"Well, I think it would be unusual for me not to include someone that I have competed against for so many years", a glum Sharapova, who has lost 19 of her 21 matches against Williams, told reporters. "I just want to play as I did today, and as I did every day".
It will be Halep's second consecutive Grand Slam final and her fourth overall attempt to win her first title in a major.
However, the US Open champion does admit that she is hard to stop when she gets her teeth into a draw. "Because it's a nice opportunity, a big challenge, so I have just to feel happy after the match that I gave everything".
Already a massive star in her home country - Halep appeared on the front cover of Romania's "Elle" magazine in April - she has received growing support from elsewhere. Muguruza is 3-0 in career grand slam semifinal matches. Like previous year, I feel Halep will make an excellent start to the match but when it gets tight, she might just falter. In the decider, Halep came to the net more often to finish points and opened up a 4-1 lead.
She won the US Open in September on a ranking of 83 and just 69 days after returning to the tour following her near year-long absence.