In the previous game, Wozniacki had tripped while sliding in for a drop shot. After being helped from the court, she had her ankle taped up and played one more game before pulling out.
Stosur, a renowned doubles player, came from 5-3 down in the second set to beat eighth-seeded Slovak Daniela Hantuchova 6-3 7-6 in their semi-final.
The Australian, who broke into the top 10 for the first time after reaching the Indian Wells semi-finals last month, powered down eight aces to complete her victory in just under two hours.
(Writing by Mark Lamport-Stokes in Los Angeles; Editing by Pritha Sarkar and Ken Ferris)