The ninth-seeded Demenetieva avenged a lopsided loss to Mauresmo earlier this year by outlasting the third seed from France, 2-6, 6-4, 6-2.
Mauresmo, who won last year's event in Montreal, was forced to take a medical timeout in the second set. She has battled knee and rib injuries this year and was playing in her first event since June.
Dementieva will have another challenge in the semifinals, where she will meet second seed Justine Henin-Hardenne of Belgium.
Henin-Hardenne easily advanced with a 6-2, 6-4 victory over No. 12 Elena Bovina of Russia.
The other semifinal will be a matchup of unseeded players as Paola Suarez of Argentina and Russian Lina Krasnoroutskaya posted quarterfinal victories.