Mason Oberg scored 18 points to lead the Titans to a road win to open the 4A bi-district tournament.
Trailing most of the game, Union took the lead with 55 seconds to go on Toryi Midland’s basket. Rebekeah Barney then blocked a potential game-tying shot by Auburn Mountainview.
“They showed a lot of character,” Union coach Mike Cranston said. “They battled. To win on the road in the playoffs at someone else’s place is tough to do.”
The Titans (16-4) advance to face Bellarmine Prep (17-3) in a quarterfinal game at 1:45 p.m. Saturday at Mount Tahoma High School in Tacoma.