Fitzner made four 3-pointers, and Emmett Naar made three more and finished with 13 points.
Joe Rahon added 14 points for Saint Mary's (9-1), which made 10 of 23 from 3-point range. It's the seventh time this season the Gaels have hit double-digit 3s.
The Gaels led by as many as 26, 70-44, after a layup by Rahon with 4:29 left in the second half and shot 50 percent from the field overall.
Saint Mary's has won 24 of the last 26 meetings with San Francisco - including a 17-game winning streak from 2006-14.
Uche Ofoegbu led San Francisco (6-5) with 13 points. The Dons were held to 17-of-60 shooting (28.3 percent).