Knicks guard Jalen Brunson celebrates with Mikal Bridges after defeating the Boston...

Knicks guard Jalen Brunson celebrates with Mikal Bridges after defeating the Boston Celtics in Game 1 of a second-round playoff series Monday. Credit: AP/Charles Krupa

BOSTON — For all of the strategy, from the offseason moves to the days of preparation, it all came down to this: the ball in the hands of the best clutch player in the game with the game on the line.

But a perfect feed from Karl-Anthony Towns to Jalen Brunson cutting to the basket resulted in his bank shot rolling off the rim. The potential game-winner in the final seconds of regulation had slipped away.

Brunson could only offer an incredulous smile as the Knicks regrouped and headed to overtime.

Maybe the 48 minutes showed them that despite what the regular season had indicated, they were up to the challenge. The Knicks didn’t back down in overtime, never trailing and hanging on for a 108-105 victory over the Celtics in Game 1 of their Eastern Conference semifinal series at TD Garden.

It ended with Mikal Bridges stealing the inbounds pass from Jaylen Brown in the final seconds to secure the win. He threw it to the other end of the court as the Knicks celebrated. A huge chant of “let’s go Knicks!” arose as the fans headed for the exits.

The Knicks had three days off to prepare for this, the chance to make a statement that the plans of summer to chase the Celtics were not what you saw in the regular season, when they lost all four games and were blown out in the first three. That now, after a full season together, what they put together on paper would pay off in the postseason.

“Yeah, man, we’re here,” Bridges said. “We’re just trying to play hard and win games. Obviously, the regular season, we just got to learn from it. Watch a lot of film and whatever we got to do to play better. But yeah, man, I think we just keep fighting."

And what they saw in Game 1 was that maybe the three players left over in the starting lineup from last season were the ones up to the task. Brunson was the best player on the floor as the Knicks turned a 75-55 third-quarter deficit into a 97-91 lead and ultimately a massive opening upset.

“I don’t think it builds our confidence,” Josh Hart said. “We always have confidence. The season is peaks and valleys. If you get too high, you’re going to get knocked down. If you get too low, you’re going to get buried.

“So I think we’ve built a tremendous amount of trust in this locker room and with the coaches, so for us to win, everybody had to play their part. And we’ll come back tomorrow and figure out to how to get another win.”

When the Knicks seemed to have little to cling to early, it was Hart who was the constant motor en route to 14 points and 11 rebounds. OG Anunoby had 29 points — 23 after halftime — and stellar defense again. Brunson also scored 29, including 20 after halftime, and Towns had 14 points and 13 rebounds.

The Celtics were undone by the weapon that had crushed the Knicks in the regular season — the three-point shot. They shot 15-for-60 from beyond the arc and Brown and Jayson Tatum were held to 23 points each. Kristaps Porzingis was scoreless in the first half and didn’t return, sidelined by illness.

The Knicks held the Celtics to 44 points and 14-for-49 shooting in the 29 minutes after halftime.

Maybe the Knicks were right all along. What happened in the regular season when the Celtics swept the four-game season series meant little in the glare of the postseason.

The missed opportunity at the end of regulation didn’t deter the Knicks as they began overtime strong. Hart found Anunoby cutting to the basket for a three-point play that gave the Knicks a three-point lead. Anunoby found Bridges for a three-pointer from the left corner and a 106-100 lead.

Brunson missed on a drive, but Towns tipped it in to up the lead to six again. Brown hit a second-chance three-pointer, and it was a one-possession game again.

Brown missed a layup, the Knicks milked the clock down and Hart had to hoist a three that missed. The Knicks fouled Brown with the clock down to three seconds before he could attempt a three-pointer and Boston called time for one more shot.

The first attempt to inbound found no one open and Boston called its last timeout. This time, as the Celtics threw it crosscourt to Brown, Bridges got his hands on it as Brown caught it and ripped it free.

Brunson silenced the TD Garden crowd with a three-pointer with 4:07 left in regulation to give the Knicks a 94-91 lead, their first since the opening minutes of the second quarter. He followed it with another three to make it 97-91 with 3:28 left.

But two free throws each by Brown and Tatum and a turnover by Brunson provided an opening, and Derrick White sank a three-pointer to give the Celtics a 98-97 lead with 2:18 left.

Out of a timeout, Brunson then fired a pass out of bounds, but Brunson found Anunoby alone in the corner for a three-pointer with 1:08 left to put the Knicks ahead 100-98. Boston tied it with 53.4 seconds remaining on a layup, of all things, by Jrue Holiday.

Anunoby missed on a jumper and White grabbed the rebound and called time on the floor before the Knicks could tie him up, giving Boston the ball with 28.1 seconds left.

Tatum tried to serve up the heroics for the Celtics, but as he ditched Anunoby on a screen, Mitchell Robinson defended him well and his three-point attempt was offline with 9.6 seconds left. Deuce McBride grabbed the rebound and the Knicks called time with 5.5 seconds left.

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