Assumption trailed Edgar 43-40 with 12 seconds before sophomore Kendall Schmidt hit a triple from the right corner to tie the game at 43 with under one second remaining.
The tying shot capped a 13-point second half comeback for the Royals.
Assumption, who earned the 2-seed in the playoffs, went on to beat 6-seed Edgar, 53-47, in overtime.
The Royals got a team-high 19 points from junior Madalyn Bohn. Fellow junior Natalie Weisenbeck tallied 10 points and Schmdit added 7 in the victory.
Edgar was led by sophomore Makenna Guden, who finished with a game-high 26 points.
Assumption will face rival Newman Catholic in a sectional semifinal on Thursday at Wausau West.