Mater Dei has upset Mission Viejo 21-14 and will advance to the Pac-5 semifinals against Long Beach Poly.
The Monarchs (10-2) intercepted three passes, blocked a punt, recovered a fumble and limited second-seeded Mission Viejo (11-1) to less than 100 yards rushing.
Chase Forrest threw two touchdown passes and Landon Iakopo ran for the decisive score for Mater Dei, which clinched the game when Koby Lowe recovered an onside kick with 42 second left.
Jonathan Lockett had two interceptions and Matthew Rockett and JacoB Welch each caught touchdown passes for Mater Dei.
"Tey've been ranked No. 1 in Orange County all year, but we played hard for 48 minutes and I think we earned our respect tonight," Mater Dei Coach Bruce Rollinson said.