Philadelphia’s James Harden shoots against Boston Celtics guard Malcolm Brogdon during the first half of Game 1 in the  Eastern Conference semifinals Monday in Boston. Charles Krupa/Associated Press

BOSTON — James Harden was asked for most of this season to be a facilitator for a 76ers team built around big man Joel Embiid.

With the MVP finalist sidelined for the opener of his team’s second-round playoff series with the Celtics, Philadelphia called on a vintage version of Harden.

He happily obliged.

Harden matched his playoff career high with 45 points and hit a go-ahead, step-back 3-pointer over Al Horford with 8.7 seconds left as the 76ers rallied without Embiid to beat Boston 119-115 in Game 1 of the Eastern Conference semifinals on Monday night.

“We didn’t come into the game expecting to lose. We came in here to win,” Harden said. “Whether Jo comes back or not, we’re going to be ready to go.”

Tyrese Maxey added 26 points and Tobias Harris finished with 18 for the Sixers, who made 17 3-pointers with Embiid watching from the bench. He sprained his right knee in Game 3 of Philadelphia’s first-round sweep of the Brooklyn Nets.

After the Sixers endured an early scoring onslaught from the Celtics, 76ers coach Doc Rivers said Harden was patient and knew when to attack and when to pass.

Rivers said having a week to prepare for the likelihood Embiid would be out was key.

“That’s the difference between this year’s team and last year’s team. We have a bunch of street fighters,” Rivers said. “If we could get to the fourth quarter, we were going to win the game. That’s how we felt.”

Jayson Tatum led the Celtics with 39 points and 11 rebounds. Jaylen Brown added 23 points and Malcolm Brogdon finished with 20.

“They hit big shots and I think they were just in a rhythm throughout the whole game,” Tatum said.

Boston hosts Game 2 on Wednesday night.

Harden previously scored 45 points for the Houston Rockets in a Western Conference Finals win over Golden State on May 25, 2015.

Trailing 117-115, the Celtics had one final chance and got the ball to Tatum, but he lost the ball to Paul Reed, who was fouled and hit a pair of free throws.

The game was tied at 87-all entering the back-and-forth final period.

Tatum gave the Celtics their final lead, 115-114, on a pair of free throws with 26.1 seconds left.

Before that, Boston took the shot clock down before Maxey intercepted a pass by Brogdon and drove in for a layup to put the 76ers in front, 114-113.

“I was about to pick the ball up, but I’m glad I kept running,” Maxey said.

Boston led 108-107 when Harden was called for an offensive foul on Horford. On the ensuing possession, Horford found Marcus Smart, who was fouled on a layup. He completed the three-point play to make it 111-107.

Maxey responded with a three-point play of his own.

The game was knotted at 102 when Brogdon got free on a fast break, took a pass from Tatum and dropped in a layup with 5:04 remaining.

After the play, P.J. Tucker appeared to swing wildly in frustration and made contact with Tatum. It led to a brief scrum that was broken up by officials.

Reed took Embiid’s place in the starting lineup, like he did with Embiid out for the final game of the Nets series. As expected, Embiid’s absence in the middle contributed to a faster pace.

Both teams used it to their advantage.

The Celtics attacked the paint, connecting on 13 of their first 14 shots. The 76ers made eight of their first 10 attempts, spacing the floor and spreading out Boston’s defense.

Harden led the attack early for Philadelphia, hitting his first five attempts, including two 3s. Brown started 6 for 6 from the field for a Boston team that thrived without Embiid protecting the rim, going on an 11-0 run at one point.

Boston shot 74% for the half but took only a 66-63 lead into halftime after the 76ers went 11 for 19 (55%) from the 3-point line.

“It was a dogfight,” Maxey said. “Everybody that stepped on the court for us was a fighter and we came out on top.”

TIP-INS

76ers: De’Anthony Melton had 17 first-half points, making each of his five 3-point attempts. According to Stathead, he joins Maxey (May 6, 2022 vs. Miami) as the only 76ers to make at least five 3s without a miss in a playoff half, since at least the 1997 playoffs. Melton was scoreless in the second half, however. … Harden scored 16 points on 7-of-10 shooting in the first quarter.

Celtics: Brown scored 14 points in the first quarter and had Tatum 20 points in the second, his highest total in a quarter of a playoff game. … Boston shot 85% (17 of 20) in the first quarter, outscoring the 76ers 26-8 in the paint.