BOSTON — In what very well could be a preview of this season’s Eastern Conference finals, the Celtics and Cleveland Cavaliers squared off in a highly anticipated thriller Tuesday night at TD Garden.

Boston is the reigning NBA champion. Cleveland looks like its top Eastern Conference challenger, entering with a 15-0 record that was tied for the second-best ever to start a season.

The result: a heavyweight bout that lived up to its considerable pregame hype.

The Celtics nearly squandered a 21-point second-half lead but recovered to win 120-117, handing the visiting Cavs their first defeat of the season and reasserting themselves as the team to beat in the NBA.

Jayson Tatum shot 50% from the floor and 60% from 3-point range to lead Boston, finishing with 33 points, 12 rebounds, seven assists and two steals. Six Celtics scored in double figures, including veteran center Al Horford, who played one of his best games of the season with 20 points, seven rebounds and three blocks.

The victory also kept alive Boston’s hopes of advancing to the knockout rounds of the NBA Cup, as a loss would have all but eliminated Joe Mazzulla’s squad from tournament contention. The 12-3 Celtics will play their penultimate game of group play this Friday at the Washington Wizards.

Cleveland had its talented top four intact Tuesday — unlike in the teams’ East semifinal series back in May, during which Donovan Mitchell missed two games and Jarrett Allen sat out all five — but played without a significant portion of head coach Kenny Atkinson’s rotation. Isaac Okoro, Caris LeVert and Dean Wade all were unavailable due to injury.

Mitchell (35 points) and Evan Mobley (22 points, 11 rebounds) had big nights for Cleveland, and the Cavs’ abbreviated bench was excellent, with Georges Niang, Ty Jerome and Craig Porter Jr. all hitting double figures. But it was a nightmare evening for Mitchell’s backcourt co-star, Darius Garland, who went 3 for 21.

The Celtics opened an early 12-4 lead despite misfiring on five of their first six shots, getting three points each from Tatum, Brown, Horford and Holiday during that stretch to trigger a Cavs timeout. Tatum powered Boston on both ends in the first quarter, notching eight points, five rebounds, three assists and two steals.

Cleveland came in as the NBA leader in shooting percentage, both from the field and from 3-point range, but it had to rely mostly on dunks and layups early in the game. The Cavaliers went 0 for 6 from three in the first quarter and scored a season-low 20 points in the frame, but they trailed by just six.

The Cavs were much better in that regard in the second quarter — but not good enough to outrun Boston’s avalanche. The Celtics made as many first-half threes (14) as Cleveland attempted and were ruthlessly efficient while doing so. The quintet of Tatum, White, Holiday, Horford and Payton Pritchard went 12 for 15 from deep before halftime — a sterling 80% — as the Celtics built a 65-48 lead.

Pritchard keyed one pivotal second-quarter sequence by scoring five points in 37 seconds out of a Cavaliers timeout. Those were part of a 10-point first half for the ascending reserve guard, who’s off to the best start of his five-year NBA career.

Three minutes later, the 38-year-old Horford denied Garland at the rim, and Tatum drilled a three at the other end to give Boston its first double-digit lead of the night. Another Tatum triple stretched that advantage to 17 points just before halftime.

The lead hit 21 early in the third quarter, with White, Holiday, Horford and Tatum all hitting additional threes in the first four minutes after halftime. Then, the Celtics went cold, and the Cavs found a path back: attacking Boston in the paint.

Despite not hitting a 3-pointer for the final 7:36 of the third quarter, Cleveland ripped off a 22-5 run to cut Boston’s lead to two. Cavs players repeatedly attacked backup Celtics center Neemias Queta, who was on the court for the entire comeback and finished as a minus-15.

Even with Queta’s struggles, head coach Joe Mazzulla chose not to give minutes to Luke Kornet. Horford and Queta were the only Boston bigs who saw action in the game.

A much-needed Tatum three at the third-quarter buzzer sent the Celtics into the fourth quarter up 93-88. White made it 102-97 with an and-one make off his own offensive rebound. After a Porter three cut the lead back to two, Tatum drove baseline for a dunk, then found Pritchard for an open three and a 107-100 lead.

Two Horford blocks and a Brown steal down the stretch helped Boston lock down the victory, with Brown adding an acrobatic layup that put the Celtics up seven with 46.1 seconds remaining.

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