Jayson Tatum sat at the dais wearing a pink collared shirt, looking relaxed and relieved and rejuvenated, and he could freely admit the truth.
“They had us on the ropes in Game 6,” he said. “They had us.”
But the 76ers were unable to take advantage of Tatum’s rough start and finish off the Celtics Thursday night. And inside the Celtics locker room, there was a powerful belief that the 76ers would never have a chance like that again. No one believed it more than Tatum heading into Game 7 of this conference semifinal series Sunday.
“I think going into Game 6, I was too — it sounds crazy — I was, like, too locked in,” he said. “I was too tight, just too in my own head thinking about, ‘What do I need to do? How many points do I got to score?’ You know, this is a big moment. And today, I was more myself.”
Being himself means laughing and smiling and revving up a lively crowd. The scoring part seemed to come easily. By the time the afternoon was complete, he had erupted for an NBA Game 7-record 51 points, guiding the Celtics to an emphatic 112-88 win.
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Guard Marcus Smart was asked what it’s like to watch Tatum when his game is flowing so smoothly.
“It’s a movie,” he said. “It’s a big movie. Being able to just sit back, eat your popcorn, and watch.”
Boston advances to the conference finals, where it will face the Heat for the second year in a row. Game 1 will be at TD Garden on Wednesday night.
Neither of these first two rounds have been easy for the Celtics, and even though the Heat have surged into the third round as a No. 8-seed, Boston knows this one will probably not be simple, either. But these tests against the Hawks and 76ers have also hardened the Celtics, and allowed them to believe in themselves more strongly than ever before.
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“In the most critical moments, they don’t overreact,” coach Joe Mazzulla said. “They trust each other, they stay together, and they execute. To me, one word to describe the series is ownership. They took ownership.”
When the 76ers flipped an early six-point deficit into a nine-point first-quarter lead, there was some uneasiness from this eager crowd that had already watched the Celtics drop two home games in this series.
But in this case, they regrouped quickly, and with the score tied at 55 pummeled Philadelphia with a 28-3 third-quarter run.
“It was beautiful to see,” forward Al Horford said. “I don’t know if I’ll go back and watch the game, I have to move ahead. But I may go back just to see what we did, because it was impressive.”
As important as Tatum’s offense was throughout the afternoon, the Celtics won this game, and this series, with defense.
Entering this round, Boston had not held an opponent below 90 points all year. The Celtics did it three times — all wins — against a 76ers offense that ranked No. 3 in the NBA during the regular season. Sunday was perhaps the finest work, and it was keyed by Horford’s constant, focused, and pesky defense on the league’s MVP, Joel Embiid.
Left to operate in single coverage for much of the game, Horford constantly forced Embiid to catch the ball far out on the perimeter. There, he’d stay low to keep Embiid from overpowering him, and keep his arms waving to keep him from getting a clean look.
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“I was just out there trying to fight for my life,” Horford said, “and trying to make an impact.”
Embiid made 5 of 18 shots and had 15 points and four turnovers. With the top of the Eastern Conference cleared after the Heat’s upset of the top-seeded Bucks, this was viewed as Embiid’s best chance to reach his first Finals. Instead, he was left to stomach yet another second-round defeat.
James Harden didn’t fare much better. It was a mercurial two weeks for the veteran 76ers guard. His last-minute 3-pointers in Games 1 and 4 may have been all that kept this from being a sweep. But his play as a whole was pretty lousy, capped by his 9-point, 5-turnover performance Sunday in which his biggest misstep sparked the game’s turning point.
With 8:26 left in the second quarter and the 76ers leading by 8, he drove and had a simple layup attempt that would have given his team the first double-digit advantage. But he lost the ball out of bounds and elbowed Celtics guard Jaylen Brown in the face as he went up, resulting in a flagrant foul.
Brown hit both free throws, Tatum found Robert Williams for an alley-oop, and then Brown rode the momentum by swooping in for a steal and layup. It was essentially an eight-point swing in 30 seconds, and the 76ers were never the same after.
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With the score tied at 55 early in the third, Tatum took over, pouring in four 3-pointers during his 17-point period that gave the Celtics an 88-62 lead. When he scored his 51st point on a 3-pointer midway through the fourth, the number was flashed on the jumbotron along with a live shot of Tatum, just in case anyone had been in the beer line for too long.
This energetic crowd responded to being challenged by these Celtics in recent days, and Tatum waved both arms through the air coaxing them to be even louder. They were happy to oblige.
“Just focused on the game and having fun,” Tatum said. “I think that’s when I’m essentially at my best.”
Adam Himmelsbach can be reached at adam.himmelsbach@globe.com. Follow him @adamhimmelsbach.