The Raptors had looked down and out trailing their Eastern Conference semi-final 2-0, with Boston half a second away from taking a 3-0 lead on Thursday before OG Anunoby sunk a last-gasp three-pointer.
Game four was less frantic, with the Raptors working well to shut down Boston's perimeter shooters, before taking control with a 32-24 third quarter.
Jayson Tatum had 24 points and 10 rebounds for the Celtics, who won their first six play-off games but have now dropped two on the bounce.
Jamal Murray finished with 27 points and Nikola Jokic had 26 points and 18 rebounds as Denver led throughout.
Paul George had 22 points for the Clippers, while Kawhi Leonard had a rare sub-par night, shooting only four from 17 in collecting 13 points, 10 rebounds and eight assists.