Fifth-seeded Saint Joseph's (21-11) can lock up its first NCAA tournament berth since 2003-04 on Saturday night in the championship game.
Despite the loss, the top-seeded Musketeers (27-6) are all but guaranteed a spot in the NCAA tournament when the selections are announced on Sunday.
Ahmad Nivins added 15 points for the Hawks, whose tough man-to-man defense limited Xavier to 37 percent shooting from the field, including only one basket outside the paint.
Josh Duncan had 11 points and Jason Love added 10 points and nine rebounds for Xavier, which was 1 of 14 from 3-point range.
The Hawks, who dominated Fordham and Richmond in the first two rounds of the tournament, took control with a 15-0 run that bridged the end of the first half and the start of the second, putting them ahead 35-20.
Calathes had a 3-point play and a 3-pointer as Saint Joseph's scored the final 10 points of the opening half. The senior forward also hit three straight 3-pointers in the second half to shortcircuit every run by Xavier.
The Musketeers only threatened once in the closing minutes, drawing to within 54-49 with 1:15 to play when Duncan made two free throws to cap an 8-1 run.
Xavier's pressure forced Saint Joseph's to call a time out, but Calathes took the ensuing inbounds pass and dribbled the length of the court for a dunk that all but iced the game.
Saint Joseph's held Xavier scoreless in the final 4:57 of the first half to take a 30-20 lead. Rob Ferguson, who had 11 points, started the run with a jumper and a layup in transition. Calathes ended it with a three-point play and a 3-point field goal, the later coming with 2.8 second to go.
The 20 first-half points were a season low, and it marked only the fifth time this season that Xavier trailed at the half. What made this one special was that the Musketeers didn't have an outside game. All eight of their baskets were on layups.
Xavier's season low was 55 in a loss at Arizona State on Dec. 15.
