The Spiders, coming off a 2-1 loss at No. 8 ranked Old Dominion on Friday in Norfolk, scored first Sunday when Deitrick netted her team-leading seventh goal of the season in the ninth minute off an assist from sophomore Rebecca Barry.
William & Mary (9-3) tied the score just four minutes later when pip Saunders scored on a penalty corner.
Davies scored her sixth goal of the season in the 24th minute and that would hold up as the game winner.
Richmond junior goalkeeper Anna Zarkoski made five saves in goal.
The Spiders (6-4) are now 2-4 against ranked teams this season. Three of their four losses are 2-1 losses to Top-20 teams.
Richmond, which improved to 4-0 at home this season, have two home games next weekend. The Spiders will host nationally ranked Massachusetts on Saturday at noon to open Atlantic 10 play and host Villanova on Sunday at 1 p.m.