Jordan Hamilton led the Raptors (4-1) with 16 points, Williams added 15, and Terrence Ross and Patrick Patterson each had 12.
Marcus Thornton scored 17 points — 12 in a 3:32 span in the fourth quarter — in 17 minutes for the Celtics (2-3). Avery Bradley and Tyler Zeller each added 13 points.
Boston scored nine straight points in the final 1:47 to tie it at 89. Bradley opened the run with a driving layup, Kelly Olynyk scored off a rebound, and Marcus Smart made three free throws with 39.3 seconds left after being fouled on a 3-pointer. After Toronto's Greivis Vasquez missed a long jumper, Bradley hit jumper with 8.1 seconds left to tie it.
Jeff Green finished with nine points in his preseason debut for the Celtics after missing the first four games because of a calf injury.
A crowd of 5,909 attended the game at Cross Insurance Arena.