BOSTON — A last-second desperation heave sent the game to overtime for the Milwaukee Bucks.

But there was no happy ending Sunday afternoon.

Al Horford had 24 points and 12 rebounds and Boston overcame Khris Middleton’s long 3-pointer that beat the fourth-quarter buzzer, topping the Bucks 113-107 in overtime to open their first-round Eastern Conference playoffs series.

The Celtics led 99-96 with 0.5 seconds left in regulation when Middleton took an inbounds pass on the far right wing and hit a contested 35-footer. But Boston outscored Milwaukee 14-8 in the extra period.

“Both teams are going to keep fighting,” Bucks coach Joe Prunty said. “That’s what this series is going to be. Each little play is going to matter.”

The game went to extra time following a wild finish in regulation that included back-to-back 3-pointers in the closing seconds.

Milwaukee was held to just two field goals over the first seven minutes of the fourth quarter as the Celtics built an 86-76 advantage.

The Bucks closed to 89-87 before Marcus Morris made a falling down jumper just inside the arc to make it 91-87.

It was 92-91 with less than a minute to play when a pair of free throws by Horford made it 94-91.

Giannis Antetokounmpo got free for a dunk. But Horford was steady again after being fouled, connecting on two more from the line with 15.2 seconds left.

Out of a timeout, Malcolm Brogdon got free on the wing and drained a 3 with 10.3 to play to tie it at 96.

Terry Rozier dribbled the clock down before crossing over Eric Bledsoe, stepping back and swishing an apparent game-winner with a half-second showing the clock.

But after a Milwaukee timeout advanced the ball to half court, Middleton matched it with his own 3 over Jaylen Brown as time expired.

“The moral of the story is we didn’t pull this one out. Got to get the next one,” Brogdon said.

Rozier added 23 points, four rebounds and three assists for Boston in his first career playoff start. Brown finished with 20 points. Rookie Jayson Tatum added 19 points and 10 rebounds.

Rozier said he knows he’s under a spotlight this postseason with Kyrie Irving out for the season following his recent knee surgery.

“I just know I gotta step up,” he said. “I know I gotta fill big shoes. I don’t feel no pressure. I’m glad to be in this position.”

Game 2 in the best-of-seven series is Tuesday night in Boston.

Antetokounmpo led the Bucks with 35 points, 13 rebounds and seven assists before fouling out in overtime. Middleton had 31 points, eight rebounds and six assists. Brogdon scored all 16 of his points after halftime. Antetokounmpo has now scored at least 20 points in eight consecutive games against the Celtics.

After a muted start, things opened up for Antetokounmpo. But he said he must do a better job on the defensive end against Horford going forward.

“He knows what he’s doing down there,” Antetokounmpo said. “Hopefully in Game 2 I won’t play on my heels and I’ll play more on my toes and be able to defend without fouling.”

Part of the reason the Bucks were in bad spots is because they were hampered by turnovers. They committed 20, leading to 27 Boston points.

“That’s not getting shots up,” Middleton said. “We feel like if we get a shot up, we’ll be good.”

The Bucks used a 26-4 run in the second quarter to take a 10-point lead.

“That was part of our plan, to kind of lull them into a false sense of security,” Celtics coach Brad Stevens said, joking.

An acrobatic, spinning layup by Tatum gave Boston a 108-105 lead in overtime.

It was 108-106 when Horford snared a rebound and got it ahead to Rozier, who was fouled. He made two free throws to put the Celtics in front 110-106 with 18.8 left.

Antetokounmpo made one of two foul shots with 14.8 seconds left, but he fouled out while going for the rebound on his miss.

Rozier added three free throws to provide the final margin.

Milwaukee 17 30 19 33 8 — 107

Boston 29 15 26 29 14 — 113

MILWAUKEE — Middleton 12-20 2-2 31, Antetokounmpo 11-21 13-16 35, Henson 3-6 1-2 7, Bledsoe 4-12 1-2 9, Snell 1-4 0-0 2, Parker 1-5 0-0 2, Zeller 1-1 0-0 2, Terry 1-1 0-0 3, Brogdon 6-13 2-2 16, S.Brown 0-0 0-0 0. Totals 40-83 19-24 107.

BOSTON — Tatum 8-18 2-2 19, Horford 5-8 13-14 24, Baynes 0-4 0-0 0, Rozier 7-18 5-6 23, J.Brown 9-19 0-0 20, Ojeleye 0-1 0-0 0, Morris 8-20 3-3 21, Monroe 0-2 1-2 1, Larkin 2-4 0-0 5. Totals 39-94 24-27 113.

3-point goals — Mil 8-21 (Middleton 5-7, Brogdon 2-6, Terry 1-1, Parker 0-1, Antetokounmpo 0-1, Bledsoe 0-2, Snell 0-3), B 11-26 (Rozier 4-9, Morris 2-4, J.Brown 2-5, Horford 1-2, Tatum 1-3, Larkin 1-3). Fouled out — Bledsoe, Antetokounmpo. Rebounds — Mil 42 (Antetokounmpo 13), B 45 (Horford 12). Assists — Mil 22 (Antetokounmpo 7), B 20 (J.Brown, Tatum, Horford 4). Total fouls — Mil 31, B 21. Att. — 18,624.