The Demons jumped out to a 1-0 lead on Maya Bauer's goal with 24 minutes left in the first half. They kept that lead until a minute left in the first half, when Emilie Wetzig scored for Avon to tie the game.
Westlake took the lead with a goal by Missy Murthy with 22 minutes left.
The sixth-seeded Demons (9-6-1) advance to the district semifinals, where they will play the winner of Thursday's game between Midview and Amherst. The game is set for 3 p.m. Saturday.