Mladenovic, who won her maiden title in St Petersburg last month, prevailed 7-5 4-6 6-2 over the American.
Mladenovic broke for a 3-2 lead but the American rallied to break back as the Frenchwoman's serve faltered. Mladenovic though regained her composure to break again before serving out the set to love.
McHale levelled the match when she broke Mladenovic in the final game of the second set.
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Mladenovic's forehand came to the fore in the final set and she reeled off three straight games to wtap up victory and set up a final against Lesia Tsurenko.
Tsurenko was leading 5-0 in the other semi-final against Mirjana Lucic-Baroni before the top seed was forced to retire handing the Ukrainian victory.
Image credit: AFP
Ashleigh Barty cruised into the semi-finals of the Alya Malaysian Open with a 6-0 7-6 (7/2) victory over Zhang Kai-Lin.
Barty's win set up a last-four clash with with Han Xinyun, who beat Wang Qiang 6-7 (4/7) 6-2 6-4.
Barty told reporters after the game: "It wasn't a breeze at all. I think I started well and I served particularly well at the start.
"I was able to get that early jump and get some early momentum but it didn't matter what the score was in the first set, Kai-Lin's a quality player, she really is, and I knew that second set was going to be a battle no matter what."
Magda Linette will play Nao Hibino in the other semi-final after getting the better of Duan Ying-Ying 7-6 (7/3) 4-6 6-1.
Hibino had earlier beaten Lesley Kerkhove 7-6 (7/4) 6-3.
US Open women
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