The 10th-seeded Stosur earned a 6-2, 5-7, 6-2 victory over Poland's Agnieszka Radwanska to reach her second WTA final of 2011.
Stosur, currently ranked 11th in the world, 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.
Competing in just her fourth tournament since being sidelined for a year due to injuries, Williams downed fourth-seeded Victoria Azarenka 6-3, 6-3 in the day's second semifinal.
The unseeded Williams earned her third Rogers Cup finals appearance, having finished as runner-up in 2000 and winning the event a year later. She secured her 38th career title at Stanford two weeks ago and is 10-0 on hard court this year.
The 2011 Rogers Cup champion will earn $360,000.