BOSTON — Round 1, not done.

One win away from securing their fifth consecutive trip to the Eastern Conference semifinals, the Celtics fell flat Tuesday night at TD Garden, losing 113-97 to the Philadelphia 76ers in Game 5.

Boston held a 13-point lead early in the second half but couldn’t hold it. The Sixers closed the gap with a cascade of 3-pointers, and the Celtics’ offense went dark during a woeful fourth quarter, during which Joe Mazzulla’s squad shot 3 for 22 from the field and was outscored 28-11.

Jayson Tatum (24 points, 8 of 19, 16 rebounds) and Jaylen Brown (22 points, 9 of 23, five rebounds) combined for just two fourth-quarter points. The Celtics shot 11 for 39 from 3-point range (28.2%), including a 4 for 12 showing from Tatum and an 0-for-4 goose egg by Derrick White, whose series-long shooting struggles continued.

The Sixers got big nights from their top stars, with Joel Embiid turning in 33 points and eight assists in his second game back from an appendectomy, and Tyrese Maxey adding 25 and 10 rebounds. Quentin Grimes was a difference-maker off the bench for Philly, scoring 18 points on 5-of-8 shooting and hitting four 3s.

The loss snapped a streak of five straight wins in home closeout opportunities for the Celtics, their last such loss coming in Game 5 of the 2023 first round against Atlanta. Now up 3-2 in the best-of-seven, they have another chance to close out the series Thursday night in Philadelphia.

If necessary, Game 7 would be Saturday in Boston.

The Celtics’ offense sputtered out of the gate, scoring just six points in the opening six minutes. That early cold spell featured as many Boston turnovers (three, including two by Brown) as made field goals (3 for 10).

But the similarly inefficient Sixers couldn’t capitalize. They held a slim 10-6 lead at the game’s first timeout and fell behind three minutes later, once Boston’s shots began falling. The Celtics pulled ahead with a 12-4 run capped by a driving Brown layup through contact.

On the previous possession, Neemias Queta failed to convert an alley-oop after catching a lob from Payton Pritchard, yet still had time to gather the ball, take one dribble and lay it in while Paul George, Maxey and Embiid stood watching.

The uncontested offensive board was one of eight rebounds Queta pulled down during his much-improved first quarter. Boston’s starting center initially avoided the fouling issues that had plagued him throughout the series before picking up two quick whistles midway through the second period, including an especially regrettable one against Andre Drummond 98 feet from the basket.

Queta’s backup, Nikola Vucevic, hit two second-quarter 3-pointers as part of a stellar start for Boston’s bench. The Celtics’ reserves — who outscored their Sixers counterparts by 40 over the first four games — put up 14 points before Philly got its first from a bench player. Pritchard, the star of Sunday’s Game 4 rout, again led that charge. He hit four of his first five shots, accounted for a third of his team’s 18 first-half assists and was a plus-16 at halftime.

Up two with just under four minutes to play in the half, the Celtics staged a 7-0 run — a Brown fadeaway, a fast-break Pritchard layup and a Brown 3-pointer — to take a 57-50 lead into the locker room.

Brown and Tatum both reached double figures before halftime, as did Embiid and Maxey. Neither team had found much success from 3-point range to that point, with Boston shooting 27.3% from 3 and Philly converting 25%.

The Celtics stretched their lead to 13 points early in the third quarter — then went five full minutes without a made field goal. During that drought, the Sixers ripped off a 15-3 run interrupted only by three Boston free throws, one of which came after Jordan Walsh stole an inbounds pass and drew a foul beneath the basket. That hustle play couldn’t halt Philly’s momentum; it scored on the next two possessions to cut the Celtics’ lead to 66-65.

Nearly that entire run came after Queta picked up his fourth foul and was called to the bench.

Boston’s offense eventually stabilized, with a midrange Brown jumper triggering a 15-points-in-four-minutes scoring binge. Tatum and Pritchard followed with back-to-back 3-pointers. Brown and Walsh grabbed steals on consecutive trips. The Celtics’ energy returned. But the Sixers did not fold.

Philadelphia hit 3-pointers on five of its final six possessions of the third quarter, then another moments into the fourth. The final make of that flurry put the Sixers in front, 88-86, for the first time since late in the first quarter.

Sam Hauser responded with two Celtics 3s, but he canceled out one with a three-shot foul on Grimes. And outside of one successfully executed lob from Brown to Queta, the rest of Boston’s offense disappeared through the final buzzer. The Celtics did not make a field goal in the final seven minutes of regulation.

Philadelphia’s lead hit double digits with 4:52 remaining. A VJ Edgecombe 3 with 2:25 to play made it 109-94 and sent Celtics fans streaming toward the exits.

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