Fitts made 12 of 20 from the field, including 4 of 8 from 3-point range, and grabbed eight rebounds for the Gaels (14-9, 5-3 West Coast Conference).
Tommy Kuhse, who added 17 points, split a pair of free throws to give Saint Mary's a four-point lead with 18 seconds left. Fitts capped the scoring with two foul shots with 10 seconds left.
Fitts' 3-pointer with 4:28 left in the game made it 77-62 for the Gaels. The Dons went on an 18-6 run to close the gap to 83-80 with 21 seconds left.
Saint Mary's took the lead for good at 5-4 and had its largest lead at 47-30 going into the break.