It happened again.

Leading by 20 points with 3:12 remaining in the third quarter, the Celtics collapsed in spectacular fashion for the second consecutive game, allowing the New York Knicks to stage another furious late-game comeback at TD Garden.

Boston made just one field goal in the final eight-plus minutes and lost 91-90 to fall behind 0-2 in the Eastern Conference semifinals.

The lone make was a coast-to-coast dunk by Jayson Tatum that put Boston ahead by one with 38 seconds remaining. But Jalen Brunson drew a foul and made both free throws on the ensuing possession, and Tatum’s off-balance would-be game-winner was blocked by Mikal Bridges as time expired.

Tatum went just 5-for-19 from the field and 1-for-5 from 3-point range in the loss, finishing with 13 points in another ugly shooting display by Boston. The Celtics shot 36.2% and were 10-for-40 from three.

The staggering loss came two nights after the Knicks erased an identical 20-point deficit to take Game 1 in overtime. In franchise history, the Celtics have dropped the first two games of 18 best-of-seven series, and they went on to lose 16 of those.

Game 3 is Saturday afternoon at Madison Square Garden.

“Our defense was great,” Jaylen Brown said. “Our offense let us down.”

Brown and Derrick White led the Celtics with 20 points apiece, but just three of Brown’s came after halftime. Kristaps Porzingis came off the bench and was limited to 14 minutes after missing most of Game 1 with an illness. He had eight points and four rebounds and was a team-worst minus-9.

Karl-Anthony Towns powered the Knicks with 21 points and 17 rebounds, with Josh Hart adding 28 points, six boards and two blocks. Brunson scored seven of his 17 points in the final five minutes. Bridges, who also iced Game 1 by picking Brown’s pocket in the final seconds, contributed 14 points and again made the decisive defensive play.

After facing criticism for their all-threes-all-the-time approach in Game 1 — when they set NBA playoff records for 3-point attempts (60) and misses (45) — the Celtics seemed to prioritize driving to the basket early in this one. Their first five points came on two Holiday layups from the dunker spot and a clear-path foul the veteran guard drew on a fast break. Al Horford followed up a miss at the rim with a putback. Brown attempted seven shots in the first quarter, and five were within eight feet of the basket, not counting a ferocious attempted posterization of Brunson that resulted in an offensive foul.

That assertiveness was necessary because Boston’s 3-point attack again lacked its usual punch. The Celtics shot just 23.8% from deep in the first half. White was the lone bright spot, going 3-for-6 on his long-range attempts and drawing a three-shot foul on another. His teammates entered halftime a combined 2-for-15 from three.

Boston also got next to nothing offensively from two of its best players during that span. Tatum grabbed nine first-half rebounds but managed just two points on 1-of-7 shooting, and Porzingis was a minus-5 in his opening six-minute shift, looking visibly fatigued both on the court and once he returned to the bench.

Brown picked up the slack by upping his offensive volume, attempting more shots before halftime (16) than he had in all but two first halves in his NBA career. His 17 points helped Boston build a 16-point first lead.

The Celtics’ defense was borderline dominant for much of the first half, save for a brief Towns-fueled run early in the second quarter. New York did not score for the first 4:51 and missed 14 of its first 16 field-goal attempts, with Boston successfully limiting the Knicks’ All-Star point guard. Brunson scored just seven first-half points, including a 3-pointer shortly before halftime.

The Celtics took a 50-41 lead into the locker room.

Up 11 midway through the third quarter, White gave the Celtics a shot of adrenaline when he took a charge to wipe out an OG Anunoby dunk. That play sparked an 11-2 Boston run that included two makes at the rim by White and a driving dunk by Luke Kornet.

Brown also drew an offensive foul on Towns and knocked down a three during that surge, which stretched the Celtics’ lead to 68-50.

Celtics head coach Joe Mazzulla then reinserted Porzingis — a surprising move given how depleted the center had looked earlier. It proved to be a smart one, however, as Porzingis immediately took a feed from White to the rack and then hit a 3-pointer on the next Boston possession. It was just his third successful three of the playoffs on 18 attempts. Porzingis later had an and-one putback dunk off a missed Tatum three to open the fourth quarter.

Payton Pritchard followed with a pair of threes, the second of which put the Celtics up 84-68 with 8:40 remaining. Then, their offense vanished.

Boston did not make another field goal until Tatum’s go-ahead dunk in the final minute. Its only points during that drought were two White free throws with 5:05 to play and two more by Tatum after New York had taken the lead on a midrange Brunson jumper with 1:59 on the clock.

The Celtics were not as hyper-reliant on the three ball during this fourth-quarter collapse as they were in their Game 1 nosedive — overall, they took 15 fewer threes than they did Monday and were well below their season average in that metric — but the shots they did take, they simply couldn’t hit. They missed 13 straight after Pritchard’s final triple (seven threes, six twos).

Tatum scored just four points in the final quarter, all in the last minute. Brown did not score after his 3-pointer at the 5:28 mark of the third and missed three shots in the final three minutes, including a floater in the paint.

Mazzulla employed a “Hack-A” strategy against Mitchell Robinson with three minutes remaining and the Celtics up four, intentionally fouling the poor-shooting Knicks center until head coach Tom Thibodeau removed him from the game. But that approach — which Mazzulla said was aimed to take Robinson, who was a plus-19 in the game, off the floor — didn’t divert New York’s momentum. The Knicks scored on their next two possessions, including an and-one finish at the rim by Towns.

The Celtics’ track record suggests they’ll deliver a strong response Saturday at what’s sure to be a cacophonous MSG. They went an NBA-best 33-8 in road games this season and have not endured a three-game losing streak since the 2023 Eastern Conference finals against Miami. Wednesday marked just the third time this season that they lost even two straight games (coming in, they were 21-2 after defeats).

Everything about these two losses was uncharacteristic, however. And now, the defending champions are on the ropes.

“We’ve just got to trust the type of guys that we have,” Celtics head coach Joe Mazzulla said. “At the end of the day, we’ve got to just find a way to win.”