Fisher went 5-of-8 from 3-point territory and grabbed five rebounds for the Wildcats (21-6, 9-5 Big East), who managed to avoid an upset only when Fisher sent the game into overtime with a clutch shot at the end of regulation.
A long three-pointer by DePaul's Jeremiah Kelly with 20.4 ticks left in regulation gave the Blue Demons the lead at 60-59. After a foul, the Blue Demons' Moses Morgan sank a pair of free throws for some insurance.
But Fisher connected on a difficult, fall-away 3-pointer with 6 seconds left to produce a tie and Villanova went on to win despite missing guard Corey Stokes.
Maalik Wayans added 17 points and Mouphtaou Yarou grabbed 15 rebounds to go with 12 points for the victors.
Kelly ended with seven 3-pointers en route to 25 points for DePaul (7-19, 1-13).