BOSTON >> James 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 Philadelphia 76ers rallied without Joel Embiid to beat the Boston Celtics 119-115 in Game 1 of their Eastern Conference semifinal matchup on Monday night.

Tyrese Maxey added 26 points and Tobias Harris finished with 18 for the Sixers, who made 17 3-pointers in the absence of MVP finalist Embiid, who sat sat out with the sprained right knee he sustained in Game 3 of Philadelphia’s first-round series against the Brooklyn Nets.

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

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 back in front, 114-113.

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.