BOSTON — Boston Celtics Coach Brad Stevens loves the postseason because it offers a chance to “bust narratives.”

The Celtics took their first step Sunday toward shattering the belief they can’t win without their biggest stars.

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

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. Boston outscored Milwaukee 14-8 in overtime.

Terry Rozier added 23 points, four rebounds and three assists for Boston in his first career playoff start. Jaylen 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’ve got to step up,” he said. “I know I’ve got to 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 will be Tuesday night in Boston.

Giannis 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. Malcolm Brogdon scored all 16 of his points after halftime.

After a muted start, things opened for Antetokounmpo, but he said he must do a better job on defense against Horford.

“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 was because they were hampered by turnovers. They committed 20, leading to 27 Boston points.

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

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 make it 110-106 with 18.8 seconds remaining.

Antetokounmpo made 1 of 2 foul shots with 14.8 seconds left but fouled out while going for the rebound on his miss.

Rozier added three free throws.

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 two baskets 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.

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, Brogdon got free on the wing and drained a 3 with 10.3 to play to make it 96-96.

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

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

“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.”