The Titans (13-18), playing on their home court, got 19 points from Kathleen Iwuoha and 17 from Tailer Butler to upset the third-seeded Mustangs (15-14).
Fullerton will play top-seeded Hawaii on Friday at Honda Center.
UC Davis 70, Long Beach State 67: Kelsey Harris scored 24 points and the Aggies (15-15) held off the 49ers (22-9).
Hallie Meneses scored 22 points for Long Beach State. Alex Sanchez missed a three-pointer at the buzzer.