Australia's Stosur, the fifth seed, made a late decision to accept a wild card into the draw pay off, notching her first victory over Azarenka in their ninth career meeting.

Stosur decided to play Carlsbad after she was bundled out of the previous week's tournament in Stanford in the second round.

"Stanford didn't quite go to plan, and I took a last minute wild card that obviously, now I'm super-grateful for," she said.

Stosur hadn't reached a quarter-final all year until this week, when she beat second-seeded Agnieszka Radwanska in the quarters before dispatching fellow wild card Virginie Razzano of France in the semi-finals.

She brushed off suggestions that with a US Open title already on her resume, her victory in this hardcourt tuneup for the final Grand Slam of the year suddenly thrust her among the Flushing Meadow favorites. But she was delighted to collect a fourth career title.

After racing through the opening set against an out-of-sorts seeming Azarenka, Stosur broke for a 4-2 lead in the second.

Facing a 0-40 deficit in that game, Azarenka saved one break point with an ace, but on the next point saw a forehand bounce back off the net cord.

Stosur kept the pressure on, muscling a forehand winner over on game point to take a 5-2 lead.

Azarenka trailed 15-30 in the next game, but held serve to force Stosur to serve for the match.

After a shaky double-fault for 15-15, Stosur edged ahead with a stinging forehand volley. She gave herself a match point with a service winner and sealed it when Azarenka fired wide with a backhand. (AFP)