World No. 134 Chang Kai-Chen stunned top seed Sam Stosur of Australia in the semifinal of the Japan Open in Osaka on Saturday.

Taiwan’s Chang overcame six double faults and hit four aces to beat the world No. 9 Stosur 6-4, 4-6, 7-6(3) in their first-ever encounter.

Stosur won the Osaka title in 2009 and was last year’s runner-up, losing to Marion Bartoli. She hasn’t claimed any titles since winning the U.S. Open last year, beating Serena Williams 6-2, 6-3.

Chang will play Britain’s Heather Watson in the final after Watson overpowered local player Misaki Doi 7-6(2), 7-5 in their semifinal earlier on Saturday.

It is a maiden career singles final for both Chang and Watson.

The prize fund for the tournament is $220,000.