A jumper by Jerome Segura cut the Fairfield deficit to 12-11 with 10:09 left in the first half, but Rider (9-6, 3-1 Metro Atlantic Athletic Conference) closed the half on a 12-2 run as the Stags (4-10, 2-2) went scoreless for more than 9 minutes and trailed 24-13 at halftime.
Fairfield made just 5 of 24 field goals, including 1 of 10 from 3-point range, in the first half.
Back-to-back 3-pointers by Shawn Valentine and Sadler gave Rider its biggest lead of the game, 44-28, with 12 minutes left.
Amadou Sidibe scored eight during a 17-7 run which pulled Fairfield within six, 51-45, with 4 minutes to play, but Rider closed the game with an 11-1 run.
Marcus Gilbert led Fairfield with 17 points. Sidibe finished with 12 and 15 rebounds.