Stosur will play for her second title in two weeks. Halep routed Anastasia Pavlyuchenkova of Russia 6-2, 6-1 to advance to her fifth final this season.
In the men's semifinals, top-seeded Richard Gasquet defeated Ivo Karlovic of Croatia 6-4, 7-5, while qualifier Mikhail Kukushkin of Kazakhstan upset defending champion Andreas Seppi of Italy 6-1, 1-6, 6-4.
Kuznetsova couldn't hold serve once in the first set, winning just six of 23 points when serving. 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.''
Stosur won her second title of the season and fifth of her career in Osaka, Japan, last Sunday. The Australian was runner-up to Caroline Wozniacki in Moscow last year.
The 18th-ranked Halep, who has won the first four titles of her career this season, jumped ahead 4-0 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. ''I moved very well and I hit the ball very strong. She didn't move very well.''
The 10th-ranked Gasquet, still vying for the eighth and final spot in the ATP Finals next month, constantly beat Karlovic's serve-and-volley play with passing shots, and broke the Croat once in each set. The Frenchman served a winner to close the match.
The 102nd-ranked Kukushkin and Seppi traded sets before Kukushkin took a 4-1 lead in the decider and served for the match at 5-3. Seppi broke back only to fall 40-love down on his serve in the next game. Kukushkin broke again when the Italian sent a shot into the net.
''I haven't played in an ATP semifinals or finals for three years, so I desperately wanted to win,'' Kukushkin said. ''In the tight end of the third set I had a stronger desire to win and it decided the match.''
Kukushikin reached his first final since winning his only title in St. Petersburg, Russia, in 2010 and is the first qualifier to reach the final in Moscow since Paul-Henri Mathieu won the title in 2002.