BOSTON — Jaylen Brown had used his public platform ahead of Sunday afternoon’s game to deliver a clear message to Celtics fans: Get loud. The energy at TD Garden for the team’s home games during its N.B.A. Eastern Conference semifinal series with the Philadelphia 76ers had been merely OK, he said.
On Sunday, Brown got what he wanted for Game 7. It was loud early, and it was loud late. The crowd cheered every dunk and 3-pointer, every defensive stop and offensive rebound.
By the time Boston’s Jayson Tatum stood near the center circle late in the fourth quarter, in the waning moments of a tour de force and the best game of his career, he beckoned the fans for even more noise. They were happy to oblige.
The crowd was still cheering when the Celtics left the court with a 112-88 victory that decided the best-of-seven series and assured Boston that its championship dream would live on.
Tatum, a first-team All-N.B.A. selection who had not exactly played flawless basketball during the series, was extraordinary on Sunday, scoring 51 points — an N.B.A. record for a Game 7. Brown added 25 points in the win. The Celtics led by as many as 30.
“That’s when I’m at my best, when I’m having fun,” said Tatum, adding that he tried to channel his childhood love for the game. “When you go out there and relax and kind of think about those days when you were at the Y.M.C.A. or whatever, the game opens up.”
A blowout loss will surely lead to an off-season of uncertainty for the third-seeded 76ers, who had title hopes of their own. But Philadelphia’s Joel Embiid, who recently collected his first N.B.A. Most Valuable Player Award, struggled in Game 7, finishing with just 15 points while shooting 5 of 18 from the field. Sixers guard James Harden scored only 9 points.
“That’s the best team in the league,” Embiid said of the Celtics. “They’re so talented, and they’ve got a lot of guys who can play great basketball. Losing to them, seven games, I thought for the most part we played hard.”
The Celtics, the No. 2 seed in the East, put the game out of reach with a searing run in the third quarter that included back-to-back 3-pointers by Brown and Tatum. The fourth quarter was a party that masqueraded as the closing stages of an otherwise tightly contested playoff series.
“When J.T. is playing like that, we’re going to be extremely hard to beat,” Brown said of Tatum.
In the process, the Celtics earned a meeting with the eighth-seeded Miami Heat in the Eastern Conference finals, starting on Wednesday in Boston. After surviving the play-in bracket, the Heat ousted the top-seeded Milwaukee Bucks in the first round, then eliminated the Knicks in six games in their conference semifinal series. Boston beat the seventh-seeded Atlanta Hawks in six games in the first round.
The Heat have a star in Jimmy Butler, who, year after year, seems to elevate his level of play in the postseason — a fearsome two-way player who seldom has an off night.
The Celtics, of course, have an explosive star of their own in Tatum, but he had his struggles against the 76ers. On Sunday, he was the best player in the building. He shot 17 of 28 from the field and 6 of 10 from 3-point range, and finished with 13 rebounds and five assists.
“We just handled the ebbs and flows of the series,” Celtics Coach Joe Mazzulla said. “We never got too emotionally high or too emotionally low. We were able to keep our emotional togetherness intact.”
Missed opportunities will haunt the 76ers, who had a 3-2 series lead with a chance to wrap it up at home on Thursday. In that game, Tatum missed 13 of his first 14 field-goal attempts. But the Celtics were solid defensively and Tatum got hot late to extend the series with a 95-86 win.
“To be honest, they had us on the ropes,” said Tatum, adding: “And I was relieved, because our season could’ve been over.”
Game 7s are inherently important, but so much seemed to hinge on this one for both teams. For the Celtics, a loss would have represented a stark regression from all that they achieved last season, when they advanced to the N.B.A. finals before losing to the Golden State Warriors in six games.
But progress is seldom linear, and the Celtics faced an unusually rocky path this season: an unexpected coaching change before the start of training camp, a season-ending injury to Danilo Gallinari before he even appeared in a game and a defense that lacked its familiar oomph.
For the 76ers, Sunday’s game, fair or not, set up as something of a referendum on the Process, the team-building exercise that, as one of its foundational pieces, landed them Embiid in the 2014 N.B.A. draft. But now was the time for a deep playoff run.
Sixers Coach Doc Rivers acknowledged the pressure before the game and anticipated the importance of his key players pushing themselves “to the max of exhaustion.”
Embiid spent his final few quiet moments before the tip dribbling near the halfcourt circle. He even hoisted a couple of pretend shots before handing the ball to his teammate Tyrese Maxey.
The rest of Embiid’s afternoon was grim. Harden’s was somehow worse. The 76ers have now made six straight playoff appearances without advancing to the conference finals.
“I thought James came to play, I really did,” Rivers said, referring to Harden. “I thought he was trying to see the game, and I thought he played downhill a lot. Where he passed the ball tonight was the right decision, and we didn’t get anything out of it.”
The series was full of uneven performances. Atop that list was Harden, who scored 45 points in Game 1 before he promptly disappeared, shooting a combined 5 of 28 from the field in a pair of losses. He resurfaced for Game 4, scoring 42 points, but was passive again in Games 5 and 6. So the question was: Which version of Harden would show up for Game 7?
He was laboring early in the second quarter when he appeared to lose his grip on the ball going for a layup. Caught in the air, Harden swung an elbow that caught Brown in the face.
“Nothing like a shot in the face to wake you right up,” Brown said.
Harden was whistled for a flagrant foul. Brown made both free throws, and then Tatum threw a lob to Robert Williams for a dunk. Rivers cited the flagrant foul as a turning point.
“After that, we never played right again,” Rivers said.
The Celtics were continuing to mount a run when Brown, who was playing with a cotton swab stuffed up his left nostril to stanch the bleeding from his collision with Harden, tumbled in front of the opposing bench. As Brown gathered himself and turned to run upcourt, the 76ers’ Georges Niang reached out from his folding chair to grab Brown’s left leg.
Brown yelled at Niang, and both players were assessed technical fouls. At the time, Boston was actually trailing. But the fans were loud, and the Celtics made sure they stayed that way.