BOSTON — Midway through the third quarter Monday night, the Celtics appeared well on their way to another lopsided win over the overmatched New York Knicks.

Instead, their Atlantic Division rivals capitalized on one of Boston’s worst shooting performances of the season, overcame a 20-point second-half deficit and pulled off a stunning upset at TD Garden, beating the defending NBA champions 108-105 in overtime to take a 1-0 lead in the Eastern Conference semifinals.

It’s the first series deficit since 2023 for the Celtics, who went 16-3 during last season’s relentless title run and dispatched the Orlando Magic in five games in the opening round. It also was their first loss to the Knicks this season after Boston swept all four regular-season meetings (including three blowouts).

Jayson Tatum and Jaylen Brown each scored 23 points for Boston, with Derrick White and Jrue Holiday adding 19 and 16, respectively. But the Celtics made just 15 of their 60 3-point attempts (25.0%) and went 24 for 97 from the field (35.1%). The latter was their worst mark of the season.

Jalen Brunson and OG Anunoby scored 29 apiece to lead the third-seeded Knicks, who went 17 for 37 from 3 to offset a 17 for 31 showing from the foul line.

The Celtics got Holiday back from the hamstring injury that sidelined him for the final three games of the opening round, but they played the final 2 1/2 quarters without big man Kristaps Porzingis, who exited with what the team called an illness and did not return.

Rotation wing Sam Hauser, who did not enter until the third quarter, also left the game with an ankle sprain. Game 2 is Wednesday night in Boston.

Tatum played the entire third quarter and was terrific, scoring 13 of the Celtics’ 26 points while adding two steals on the defensive end. He found and exploited favorable matchups against Knicks big man Karl-Anthony Towns, with three of his first four looks against the Knicks big man resulting in a made 3-pointer and two trips to the foul line. Tatum also revved up the Garden crowd with a transition dunk off a long-range outlet pass from Brown.

The Celtics were effective at drawing fouls in the early going — New York was whistled for five in the first five minutes, including two each on Towns and Josh Hart — and at working the offensive glass. They grabbed seven offensive rebounds in the first quarter alone, leading to 10 second-chance points.

Boston needed those wins on the margins, because all of their non-Tatum players were struggling to find the basket. The rest of its roster went a combined 3 for 16 from the field and 2 for 9 from 3 in the opening quarter, including oh-fers from Holiday (on two shots), Brown (three) and Porzingis (four). As a team, the Celtics started 2 for 9 in the paint and 2 for 6 inside the restricted area.

Pritchard provided some valuable supplementary scoring with two free throws and a made 3-pointer in the final 70 seconds of the first, helping buoy the Celtics until the rest of Tatum’s supporting cast caught up.

They did so early in the second quarter. Eleven combined points from Brown and Holiday and two more buckets from Pritchard allowed Boston to not just maintain, but extend its lead while Tatum watched from the bench. The Celtics, who led 36-35 after one, outscored the Knicks by seven points during Tatum’s six-minute respite, the last two of those coming on a strong offensive board and drawn foul by Al Horford.

Boston received nearly as much help from a member of the Knicks roster during that stretch.

Backup big man Mitchell Robinson is a useful rim protector and offensive rebounder, but he’s an abysmal free-throw shooter, entering Monday with a success rate of 26.7% this postseason and 52.2% for his career. The Celtics exploited that weakness by sending Robinson on the line eight times in the first half. He went 2 for 8. Eventually, New York head coach Tom Thibodeau instructed Robinson to intentionally foul a Celtics player so he could replace him with Precious Achiuwa, who hadn’t played since April 13.

Brown scored the final six points of the first half to send Boston into the locker room up 61-45.

The Celtics’ guards then assumed control in the third quarter, taking turns drilling 3-pointers and flustering Towns. Holiday hit one 3 and drew a charge on the Knicks big — Towns’ fourth foul of the night. White assisted on Holiday’s triple, hit three of his own in a three-minute span and denied Towns with a block at the rim.

Horford, who started the second half in place of Porzingis, also blocked one of Towns’ shots and turned White’s rejection into a fast-break dunk that put Boston up 72-52.

The Celtics had ample opportunities to put the game out of reach, but they missed open 3-pointers on four consecutive possessions, and New York stormed back. Boston scored just nine points over the final 5:47 of the third quarter — including back-to-back 3s by Tatum, his first points since the first quarter — and led by 11 entering the fourth.

Two minutes and six missed shots later, that lead was down to one, 84-83. A driving dunk by Anunoby off a Tatum turnover tied the game with 7:25 to play.

Horford — Boston’s only available rotation big by that point with Porzingis in the locker room and Luke Kornet sitting with five fouls — then blocked Towns at the rim for a second time, springing Holiday for a much-needed corner 3 at the other end.

The momentum shift was short-lived. Brunson scored on New York’s next three trips down the floor, drawing a foul on Brown and hitting two 3s, the second of which put the Knicks ahead 94-91. Then, after a Celtics timeout, he hit another.

Down six with 3:28 remaining, Boston regained its poise, setting the stage for a thrilling finish. Brown drew a shooting foul on Mikal Bridges and made both free throws. Tatum blocked Anunoby’s midrange jumper and drew a whistle on the ensuing fast break. Brunson was called for a double dribble. Holiday drove and kicked to a wide-open White, who nailed a 3 that put the Celtics back ahead, 98-97.

Anunoby responded with his sixth 3-pointer of the game, one shy of his season high. Holiday punched back with a tying layup. Brunson’s high-arcing floater over a fully extended Horford missed by centimeters, and a turnaround Tatum jumper at the buzzer came up short, sending the game to overtime.

New York scored the first five points of the extra session — including a 3-pointer by Bridges, the big-ticket offseason pickup who struggled in several of the team’s regular-season meetings — and held Boston scoreless for nearly three minutes. The Celtics pulled within three on a 3-pointer by Brown and had a chance to equalize in the final seconds, but Bridges picked Brown’s pocket before he could attempt a shot, sealing the win for the underdog visitors.

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