But instead of turning into a formality, the rest of the game turned into a surprising mess for Boston. The Knicks erased that entire deficit, capitalized on the Celtics’ woeful 3-point shooting, and eventually secured a 108-105 overtime win when Mikal Bridges ripped the ball away from Jaylen Brown on the final possession.

The Knicks, who needed some good fortune to escape a gnarly first-round series against the Pistons, now hold a 1-0 series lead and have wrestled homecourt advantage away from the Celtics. It is the first time the Celtics have trailed in a series since the 2023 conference finals.

It is certainly not time for Boston to panic. The Celtics swept the season series and were still in this game in the final seconds despite a miserable 15-for-60 effort from the 3-point line. But this result certainly adds some urgency for Game 2 on Wednesday.

“[In the first round] Orlando kind of took the 3-point line away,” said Brown, who was 1 for 10 from beyond the arc. “I felt like tonight they were kind of daring us to shoot. They wanted us to shoot those shots. And we didn’t make them.”

The Celtics made just 34 of 97 (35.1 percent) shots overall, their worst output of the season. During the regular season Boston was just 1-10 with shooting 40.5 percent or worse. In the third quarter Monday, 19 of Boston’s 20 shots came from beyond the arc, and they did not attempt a free throw.

It was a stark contrast from the first half, when they were 15 for 25 on 2-pointers and attempted 20 foul shots.

“Probably some times that we settled,” forward Jayson Tatum said. “I think, for myself, I could have put more pressure on the rim, but there’s a lot of times we felt like we got some really, really good looks and just couldn’t convert.”

The Celtics, of course, set an NBA-record for made and attempted 3-pointers during the regular season, and many of those second-half tries were wide open. It’s not reasonable to expect them to just stop firing, but coach Joe Mazzulla acknowledged there were probably 5-10 shots that required better execution.

Brown and Tatum scored 23 points apiece to lead the Celtics but combined to go just 14 for 43 from the field. Celtics center Kristaps Porzingis left the game because on an illness early in the second quarter and did not return.

“I think we felt [Porzingis’s absence, but it’s no excuse,” Mazzulla said. “We had plenty of opportunities to do it, and hopefully he’s ready for Game 2.”

Sam Hauser sprained his ankle late in the third quarter and missed the rest of the game.

The Knicks, meanwhile, managed to escape with a win despite their own imperfections, such as hitting just 17 of 31 free throws. Of course, their 11-for-19 effort from beyond the arc in the second half was one effective way to hide other blemishes. Jalen Brunson and OG Anunoby scored 29 points apiece.

The Knicks trailed, 72-52, midway through the third quarter, and with Karl-Anthony Towns on the bench after collecting his fourth foul, their outlook appeared bleak.

But the Knicks whittled the deficit to 84-75 by the end of the period and then held Boston scoreless for nearly four minutes to start the fourth.

Consecutive 3-pointers by Brunson gave the Knicks a 97-91 lead with 3:27 left, their first since early in the second quarter. The Celtics responded with a 7-0 burst, but those good vibes proved to be fleeting. Tatum missed three 3-pointers and a 20-footer over the final 100 seconds of regulation.

Brunson got a near-perfect look at a running layup in the final second, but it rimmed out, leading to overtime.

New York quickly seized control in the extra session, however, with an Anunoby 3-point play followed by a Bridges 3-pointer giving the Knicks a 106-100 lead.

Brown pulled Boston within 108-105 on a 3-pointer with 1:16 left but missed a tough layup on the next possession. New York’s Josh Hart’s 3-pointer with 18 seconds left was off, and after Derrick White grabbed the rebound Mazzulla let his team play on, in part to keep poor defenders Towns and Brunson on the floor.

But that possession went nowhere. Mazzulla said he tried to call a timeout with about eight seconds remaining, but it was not granted. Towns eventually used the Knicks’ foul-to-give with three seconds left. The Celtics then called consecutive timeouts, but after White fired a crosscourt pass to Brown, Bridges ripped it away.

Boston’s locker room emptied quickly afterward. Most often, the televisions would be turned to the other playoff game involving the Nuggets and Thunder. But in this case, they were both off. At one point, White was the only player there. He eventually stood from his chair and started loudly clapping, seemingly eager to get back onto the court and try again.

Adam Himmelsbach can be reached at [email protected]. Follow him @adamhimmelsbach.