Halep won the match after three sets, ended 7-6, 3-6, 6-4.
Svitolina rallied briefly to get back to 5-3 but then handed Stephens another break and the first set when she sent a forehand wide.
The third-seeded American has yet to drop a set in her three matches at the tournament.
But Halep did not lose a point on her serve the rest of the set, and broke twice more to open the second set.
"I thought I played well, I thought she played well, " Stephens said.
Stephens broke serve in the opening game and took a 4-0 lead before Svitolina found the range on her groundstrokes.More news: Space Force: Trump 2020 asks supporters to vote on logo
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Halep had two further chances to claim the victory Stephens' serve, only for the American to produce further impressive resistance to hold.
"She's moving O.K. I just have to talk to my coach before the match, trying to stay stronger on my legs, and trying to be with the energy at the highest level as possible". However, she had an easier time against the No.19 who beat her in Zhuhai, before surrendering to Goerges in the semifinals, and forced her to a final set tiebreak in the quarterfinals en route to her first Grand Slam title at Us Open.
She had nothing to add to her scheduling dispute, which saw her play four matches in three days after a night session was postponed due to rain.
"That's a great thing for me". (On Thursday), I played two matches. "I'm happy to be back in North America and playing on hard courts, and I'm hoping for some more good results", said Stephens. I'm happy to be playing good tennis and get better throughout the tournament. It certainly lived up to expectations, as Roland Garros victor Halep fended off four set points to claim the opening set.
"(On Saturday) I'm going to go on court and I will try to give my best. Making the finals here is incredible. "I think the match was a very good level of tennis even if the rhythm was a little bit mixed".
After Stephens won the second set, Halep took control in the third, jumping out to a 5-2 lead. She broke again for a 4-1 lead, but the Australian then had her best moments, breaking back after a cross-court shot that drew a roar from the crowd. "It was a nice to be at home just cruising with my family, seeing them after I was away for two and a half months".