The Tigers (4-0) shot poorly from the perimeter early and had a hard time shaking the new-look Musketeers (2-2), missing badly on a shot to win it in regulation.
Auburn took control in the overtime behind its defense, outscoring Xavier 11-2. The Tigers scored 31 points off Xavier's 22 turnovers overall to earn a spot in the semifinals against the Duke-San Diego State winner.
Auburn had a five-point lead with a minute left in regulation, but Naj Marshall made a 3-pointer from the wing to pull the Musketeers within 77-75.
Xavier tied it at 77-all with 26 seconds left in regulation on Tyrique Jones' two free throws and Brown's last-second shot came up well short.