Down 2-1 in the Eastern Conference semifinals against the Boston Celtics, the Philadelphia 76ers needed a win on Sunday. When a comeback victory for the Celtics looked inevitable, James Harden answered the call.

His 42-point explosion included a jumper with 16 seconds remaining in regulation to push the game to overtime, along with the game-winning 3 with 19 seconds left in OT.

The 76ers were up 16 points in the second quarter, the team's biggest lead of the series. But the defending Eastern Conference champions outscored Philadelphia 24-15 in the fourth to tie the contest, 96-96.

Still clutch, Harden sealed the 116-115 win from the corner. The shot was assisted by Joel Embiid, who dished the ball while being double-teamed.

The moment was big, but Harden had some extra motivation in the stirring performance.

Keeping a promise, Harden hosted fan John Hao for the game. Hao was paralyzed in Michigan State's on-campus mass shooting in February, which killed three students.

Celtics guard Marcus Smart almost countered to win the game, but he didn't get the shot off in time.

Rough first half for Tatum and Celtics

Four minutes into the game, the Celtics had seven points and two fouls, all from Jaylen Brown. On the other hand, Jayson Tatum was 0 for 5 with zero points and six rebounds.

The 76ers took advantage of the Celtics' dormant stars to take a 27-18 lead in the final minutes of the first quarter, thanks to 12 points from Joel Embiid and six offensive rebounds from Tyrese Maxey. Philadelphia ended the quarter up by eight points.

Embiid recorded 34 points, 13 rebounds and four assists in the game. Maxey had eight rebounds, 14 points, two steals and a block.

After Embiid led the first, Harden took charge in the second. He scored 12 points in the second quarter.

Tatum scored his first points of the game with less than 30 seconds remaining in the half. On the other end of the court, Harden scored his 21st point of the game. The 27-foot step back was his third made 3, making him perfect from beyond the arc in the first half.

The Celtics went on an 8-0 run in the final minutes of the third, snapped by an impressive drive from Maxey. Tatum started to heat up, scoring 13 points on 5-for-5 shooting in the third. The Celtics closed the quarter down 92-83, but matched the Sixers scoring. Both teams put up 33 points in the quarter, a shift after the Celtics were outscored in the previous two periods.

A 24-foot 3-pointer from Smart halfway through the fourth brought the Celtics within five points, then this ridiculous series happened to tie the game:

Tatum's late surge resulted in a total of 24 points, 18 rebounds, six assists and four steals. Brown and Smart scored 23 and 21 points, respectively.

Tatum was 1 of 6 from deep. His lone 3-pointer came with 26.4 seconds remaining in overtime and appeared to come after she shoved Maxey for some extra space. 76ers head coach Doc Rivers didn't hold back when he spoke about it after the game, calling the missed call "awful."

Before the final possession of overtime, Celtics head coach Joe Mazzulla had an opportunity to call a timeout. After the game, he told reporters there was no need.

Mazzulla and the Celtics will look to regroup before they host the 76ers for Game 5 of the series on Tuesday.