Boston were missing two starters for Game 1 on Tuesday night with Al Horford entering health and safety protocols and Marcus Smart out with a foot sprain.
Derrick White came into the starting five for Ime Udoka but he could not have the same effect as 2022 Defensive Player of the Year, Smart. As a result of the change, sharpshooter Payton Pritchard played 30 minutes off the bench, dropping a solid 18 points - including a few deep three-pointers.
Horford's replacement, Grant Williams III, made his first appearance since Game 3 of the Conference semi-finals. The 24-year-old had 18 points, 9 rebounds and 2 blocks.
The Celtics had the better of the first-half with Jayson Tatum continuing his momentum from the previous series against the Bucks, dropping 21 points before the break. Boston went into the half up eight points and looking very good, with Aaron Nesmith setting the tone with a huge block on Caleb Martin.
However, the Heat, and Jimmy Butler in particular, came out swinging in the third.
They were excellent on defence, limiting the Celtics to two shots made in the whole third quarter as Butler and co. consistently stole the ball. Three minutes into the third, the Heat were in front and they didn't look back.
Butler took over in the second half, scoring 27 of his 41 points during his team's comeback. With Tatum struggling from the field, the former Chicago Bulls man was teaching the Celtics star a thing or two, draining an incredible turnaround jumper to beat the shot-clock, making it clear that it was Miami's night.
Celtics shooting guard Jaylen Brown plucked away at the hosts' lead with some big threes and Daniel Theis had a monstrous block to deny Dewayne Dedmon, but it was not enough to spark a comeback of their own.
Off the back of Butler's 41, Tyler Herro's 18 and Gabe Vincent's 17, the Heat maintained home court advantage as they battled off Tatum's 29, Brown's 24 and Williams' 18 with the Celtics falling to a 118-107 loss in Game 1.
Game 2 will tip off on Thursday night as the 2-seed and 1-seed lock horns again.