The Billikens fell back into a tie with Virginia Commonwealth at the top of the Atlantic 10 Conference with one game to play.
Saint Louis will host La Salle on Saturday and Virginia Commonwealth will visit Temple on Sunday.
There were nine nationally-teams in action Wednesday and St. Louis was one of the four that lost.
Jordan Jett tipped in a shot for the Billikens with 8 seconds left in regulation to force overtime, but Xavier pulled away during the extra 5 minutes. The Musketeers took the lead for good on a layup by Jeff Robinson with 3:29 left in overtime and they went on to end Saint Louis' 11-game winning streak.
Christon scored 20 points and Travis Taylor had 19 for Xavier (17-12, 9-6 A-10).
Saint Louis (23-6, 12-3) got 17 points from Kwamain Mitchell and 16 from Dwayne Evans.