Jimmy Butler scored 27 of his 41 points after the break, and Miami's defense locked down Boston to flip an eight-point halftime deficit into a 17-point lead by the end of the third quarter in a 118-107 conquest.
The Celtics lost Al Horford to health and safety protocols in the hours before the series started and ruled Marcus Smart out with the right mid-foot sprain he suffered in Game 7 of their second-round playoff series. Whether they ran out of gas two days after their hard-fought battle with the defending champion Milwaukee Bucks or succumbed to Miami's relentlessness, the Celtics collapsed after halftime and never recovered.
The Heat, who were also without Kyle Lowry (hamstring), found scoring from Tyler Herro (18 points, eight rebounds), Gabe Vincent (17 points) and Max Strus (11 points). Their defense changed the course of a game that nearly got away from them, amassing 12 blocks and forcing 16 turnovers (turned into 19 points).
The Celtics also lost center Robert Williams III with six minutes remaining in the game. He experienced cramping in his left leg, which also required meniscus surgery in March and suffered a bone bruise against the Bucks. After missing the last four games of the conference semifinals, Williams returned to the starting lineup against the Heat and anchored a defense that slowed Miami early, staking Boston to an 18-9 start.
Herro, the Sixth Man of the Year, settled the Heat's offense, totaling seven of his points and three of his assists in the first quarter to withstand Boston's early onslaught and keep their deficit respectable, 28-25.
Jayson Tatum scored 21 of his 29 points in the first half, when the Celtics led by as many as 13 points and 62-54 at the break. But Miami never quit. Butler and Bam Adebayo totaled seven points on a 10-1 run to start the second half, giving the Heat their first lead since they inched ahead for 16 seconds in the first quarter.
Butler inspired a stifling defensive effort after Boston shot 59% from the field in the first half. Two straight steals led to four of his 11 points during Miami's 22-2 start to the third quarter. The Celtics trailed, 76-64. When the third quarter was done, Boston had eight turnovers and just two field goals on 15 attempts.
The Heat's lead reached 96-76 on three Herro free throws to start the fourth quarter. Boston was lost. Jaylen Brown could not find any seams in the defense, and the Celtics were relying on Payton Pritchard, Daniel Theis and Aaron Nesmith off the bench — three players on the fringes of their healthy rotation.
The Celtics scratched back within nine points with seven minutes remaining and 114-107 just inside of two minutes, but Butler and the Heat had an answer for every run. His block of Pritchard erased Boston's final hope, and his last-minute layup gave him the fifth 40-point game of his playoff career and third this year.
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