Stosur proceeded to her second straight final by beating her doubles partner Svetlana Kuznetsova 6-2, 6-4, while Halep routed Anastasia Pavlyuchenkova 6-2, 6-1 to advance to her fifth final this season.
Kuznetsova couldn't hold serve once in the first set, winning just six of 23 points when serving, while Stosur was broken twice as well. Stosur took a 5-1 lead in the second but then failed to serve out the match twice.
"At the start we were both struggling to hold serve and she had love-40 again on me and my second service again and I was able to get out of that," Stosur said. "And I think that gave me a little bit of a head up and to win that first set 6-2 was good. It was still pretty tight all the way through with who was going to hold."
Stosur sealed the victory on her second match point when the Russian sent a shot wide.
"We know each other's games pretty well. So I guess you've got to take your opportunities when you get them," she said.
Stosur won her second title of the season and fifth of her career in Osaka, Japan, last Sunday. The Australian was a runner-up to Caroline Wozniacki here last year.
The 18th-ranked Halep, who has won the first four titles of her career this season, jumped out to a 4-0 lead in both sets against Pavlyuchenkova. The Romanian sealed the victory on her second match point when Pavlyuchenkova double-faulted.
"The score, it looks like it was very easy, but it wasn't," Halep said. "Today I moved very well and I hit the ball very strong. She didn't move very well today. Of course I'd like to win my year's fifth final but it will definitely be a hard task as Samantha (Stosur) is a top player in a good form."