PHILADELPHIA — For a few fleeting moments Sunday, the scoreboard hanging high above the Wells Fargo Center court may have given the Celtics fans sprinkled throughout the arena some hope.
It showed that Boston had escaped Game 4 with a 2-point win thanks to a Marcus Smart 3-pointer at the overtime buzzer. It was enough to cause some confusion and unease among the 76ers crowd that had spent the past half-hour stressing out about their team’s collapse.
But the players on the floor seemed to know the truth, and once a quick review was done to verify that Smart’s shot had come after the final buzzer, once the scoreboard changed to show 76ers 116, Celtics 115, and once the public address announcer provided the final confirmation, roars were mixed with exhales.
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By a fraction of a second, the Celtics had missed out on a chance to seize complete control. Now, they return to Boston for Tuesday’s Game 5 of this Eastern Conference semifinal a 2-2 tie.
At best, the Celtics will have to return to Philadelphia and attempt to win this series in six games. At worst, this could be the start of a fateful tailspin.
“Both teams are physically tired, mentally tired,” Celtics forward Jaylen Brown said. “And we would’ve liked to be up 3-1, but we’re not.”
After an uninspiring first half in which forward Jayson Tatum was held to 2 points, it seemed that the Celtics had regrouped and done enough to overpower the 76ers, who appeared fatigued and frustrated as Boston roared back from a 16-point deficit.
But the Celtics were unable to finish. With the score tied at the end of regulation, Tatum drove and found Smart for a wide-open 3-pointer that caromed off the rim.
After a Tatum 3-pointer with 38.1 seconds left in overtime — his first of the game — gave Boston a 115-113 lead, a defensive lapse crushed the Celtics.
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James Harden, the Game 1 star who had crumbled in his team’s last two losses, was in the midst of another vintage performance. And as he stood in the right corner with time running down, Brown left him to help double-team 76ers center Joel Embiid in the post.
Embiid sprayed a pass to Harden, who calmly drilled the game-winning 3-pointer.
“Just a bad read,” Brown said. “It’s a gamble at the wrong time. Big shot by James Harden, but that’s my fault. I take full accountability. Just a bad read.”
Celtics coach Joe Mazzulla did not call a timeout prior to the final play of regulation, and he did not use one at the end of overtime, either. This approach is favored because it keeps the opponent from setting up its defense, but in this case Boston’s final play simply took too long to materialize.
Tatum stood just a few feet inside midcourt with both hands on the ball and 5.6 seconds remaining. He eventually took four dribbles to the left arc, and with 2.7 seconds left a double-team arrived. It looked like he had a good angle for a layup as he attacked, but instead he jumped and whipped a pass to Smart, who caught the ball with 0.3 seconds left. By the time the guard rose and fired, the buzzer sounded, wiping away what would have been the biggest shot of his career.
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“I waited a second too late when they came and doubled,” Tatum said. “When Embiid came over, I tried to kick it out, but I probably should have went a dribble or two earlier.”
Mazzulla said he was pleased with the play, which gives Tatum a choice of whether to shoot or find an open man.
“We just had to play with a little bit more pace,” Mazzulla said.
The night provided sweet redemption for Harden, who had followed up his 45-point masterpiece in Game 1 by going 5 for 28 in the last two losses. He was booed for much of Game 3, when he also coughed up five turnovers and became unusually reluctant to take open shots. But on Sunday, Harden was active and aggressive and did things his way, finishing with 42 points on 16 of 23 shooting, 9 assists, 8 rebounds, and just 1 turnover.
Embiid had 34 points and 13 rebounds, but the MVP appeared fatigued during the fourth quarter, when he made 1 of 6 shots and was constantly pushed back by Celtics forward Al Horford.
The 76ers scored 4 points over the first eight minutes of the fourth, as their crowd’s energy shifted from joy to unease. The Celtics used a 10-0 run to take their first lead since the first quarter, 98-96, with a Horford dunk and ensuing shimmy capping it.
But Harden forced overtime with a tough runner with 15.9 seconds to play, and he was not done yet.
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Tatum shook off his quiet first half and finished with 24 points and 18 rebounds. Brown added 23 points, but just 11 came after the first quarter.
“We knew we were going to get their best shot today,” Horford said. “And they put it out there. We were able to withstand it and just weren’t able to close the game.”
Adam Himmelsbach can be reached at adam.himmelsbach@globe.com. Follow him @adamhimmelsbach.