The No.7-seeded Stosur double faulted twice to lose serve in the opening game but went on overdrive from there, winning 11 of the next 13 games to build a 62 51 lead. The No.8-seeded Kuznetsova won the next three games to close to 62 54, but Stosur broke one last time to close the match out, 62 64.
"At the start we were both struggling to hold serve, and she had 0-40 on me in my second service game and I was able to get out of that, and I think that kind of gave me a little bit of a head up," Stosur said. "We know each other's games pretty well, so it was still pretty tight all the way through with who was going to hold, but I think it was just about taking my opportunities when I got them today.
"Towards the end there I kind of let her back in - she was playing better and didn't make so many unforced errors in those last few games - but it's also pretty tough to play someone who you're friends with and know pretty well, and play doubles with as well. So I'm happy I managed to get through it."
"I felt a little bit low energy today but Sam was serving well and playing well, so good luck for her in tomorrow's final," Kuznetsova said. "From my side I hope to have more energy for next time. My level dropped like 25%, which wasn't enough to complete against a player who plays as well as Sam does."
Stosur has now won nine matches in a row - five to win the title in Osaka last week, and four to reach the final here in Moscow this week. She is through to her 18th WTA final, going 5-12 in her first 17.
"It's been a great couple of weeks, so hopefully I can just keep it rolling for one more match here."
The No.5-seeded Halep followed Stosur into the final with a 62 61 win over Anastasia Pavlyuchenkova.
Halep is now through to her eighth WTA final, but her 4-3 record in her first seven can be split into two distinct sections - she was 0-3 in WTA finals before this year and 4-0 in WTA finals so far this year, capturing her first four WTA titles at Nürnberg, 's-Hertogenbosch, Budapest and New Haven.
Stosur leads Halep in their head-to-head series, 3-1, though the pattern is similar to Halep's finals - Stosur won all three of their meetings before this year, while Halep won their only meeting this year.