Isaiah Flynn scored 18 of his 22 points in the second half and helped the Lions pull away from the Rams.
“Just the way the game was flowing,” said Flynn, who keyed a 10-2 run that turned a two-point lead into a 52-42 advantage early in the fourth quarter.
Bellarmine Prep forwards Sefo Liufau and Lucas Meikle terrorized Mount Rainier in the paint. Liufau totaled 20 points, 22 rebounds and a team-high six assists, and Meikle finished with 17 points and nine rebounds.
The Lions out-rebounded the Rams 39-19.
Mount Rainier guard Malik Rogers scored 15 points, guard Adrian Davis added 13 and Caden Rowland chipped in 10.
Bellarmine Prep will meet top-ranked Davis in the semifinals at 7:15 p.m. Friday. The Lions watched part of the Pirates’ 79-45 win over Bothell.
“They play as advertised, seemed like the play real well together,” Bellarmine Prep coach Bernie Salazar said. “Need to get to work now and study them (on video).”
Mount Rainier meets Bothell in a loser-out game at 12:15 p.m.