Stosur, seeded seventh, opened the semifinal round with a 6-2, 6-4 win over No. 8-seeded Svetlana Kuznetsova and Halep, the No. 5 seed, followed with a 6-2, 6-1 decision over Anastasia Pavlyuchenkova.
Stosur piled up seven breaks in nine receiving games in her win over Kuznetsova. Stosur took 60 percent of the points overall off Kuznetsova's serve while winning just more than half -- 38-of-75 -- of the points on serve.
Halep had six breaks in her win over Pavlyuchenkova. She won nearly 70 percent -- 12-of-16 in the second set -- of the points while serving despite one dropped game.
Stosur has a 3-1 advantage in previous matches with Halep claiming her first win in the series this year in Cincinnati in a three-set battle.
Halep is looking for her fifth WTA title of the year. She notched her first championship in mid June and had another the next week. She added wins in July and August in improving to 4-3 in WTA finals.
Stosur is 5-12 in finals appearances, including a win last week in Osaka, Japan. She also won in California in August, which was her first title since she won the U.S. Open in 2011.