Australia's Stosur entered the Carlsbad title tilt with a 0-8 all-time mark against the 24-year-old Belarusian, but dominated Azarenka 6-2, 6-3 Sunday at La Costa Resort and Spa.
The fifth-seeded Stosur turned aside six of Azarenka's seven break-point chances in the first set and five more in the opening game of the second set in posting her first win since the 2011 U.S. Open.
The Aussie improved to 4-12 all-time in finals.
Despite coming up short in her bid for a 17th career victory, Azarenka is expected to wrest the world No. 2 ranking from Maria Sharapova this week.
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