Halep also thrashed the world number 22 two weeks ago in Stuttgart and there was never any danger of an upset as the Romanian reeled off five straight games to close out the opening set.
Sevastova suggested resistance early in the second as she raced into a 3-0 lead.
But it was short lived as Halep reeled off six games to wrap up victory in 73 minutes.
Mladenovic, 23, moved into her fourth final of the season following a straight sets win over the veteran Russian Svetlana Kuznetsova. The 14th seed dispatched the ninth seed 6-4 7-6.
Mladenovic won her first title on the circuit in St Petersburg in February and following her strong performances in Acapulco and Stuttgart, will rise to the top of the list for the women’s season ending championships in Singapore if she prevails against Halep on Saturday on the centre court in Madrid.
“It’s true I am well placed in the race,” said Mladenovic. “It’s starting to become an important thing but I can’t get too focused on it because it is a very long season.”