The 10th-seeded Stosur earned a 6-2, 5-7, 6-2 victory on Saturday to reach her second WTA final of 2011, and will play the winner of the day's second semifinal between Serena Williams and Victoria Azarenka.
Stosur lost to Maria Sharapova in the Rome title match back in the spring and on Sunday will try to improve upon her 2-8 mark in WTA finals. Her last title came in the spring of 2010 at Charleston.
Currently ranked 11th in the world, Stosur had a breakout campaign last year with a surprising runner-up finish at the French Open and a quarterfinal appearance at the U.S. Open. She's had a disappointing 2011 season that has featured third-round losses at the Australian Open and French Open, along with a first-round setback at Wimbledon.
Stosur breezed through the first set Saturday in a mere 24 minutes and broke serve to start the second. Radwanska quickly countered with a break in the next game and earned a second break in the final game of the second set, which took nearly an hour to complete.
After falling behind 2-1 in the third, Stosur ran off the next five games to prevail.
Radwanska had a nine-match winning streak stopped. She was bidding for a second straight WTA title, having won last week in Carlsbad.
The 2011 Rogers Cup champion will earn $360,000.