The third-seeded Musketeers (21-11) will face second-seeded and 14th-ranked Creighton in the semifinals on Friday night. The Bluejays advanced with an 84-62 victory over DePaul. The Muskateers action will be on 55 KRC at 9-pm.
Both Creighton and Xavier were making their debuts in the Big East tournament.
Matt Stainbrook scored down low with 4:32 to play to give Xavier a 57-51 lead and that would be the Musketeers' last field goal of the game as they went 11 of 14 from the free throw line to hold off sixth-seeded Marquette (17-15), which lost its fourth straight.