Celtics forward Jayson Tatum drives against De’Anthony Melton of the 76ers during Game 7 of the Eastern Conference semifinals Sunday in Boston. Tatum finished with 51 points, leading the Celtics to a 112-88 rout. Steven Senne/Associated Press

BOSTON — Jayson Tatum gave Boston a long-overdue fast start and followed it up with a finish the NBA had never seen before.

After struggling in back-to-back games, Tatum erupted for 51 points – the most ever in a Game 7 – and the Celtics beat the Philadelphia 76ers 112-88 on Sunday to advance to the Eastern Conference finals for the second straight year.

One game after missing 14 of his first 15 shots, only to awaken in the fourth quarter in time to save Boston’s season, Tatum scored 25 points in the back-and-forth first half and 17 more in Boston’s 33-10 third quarter that turned a three-point lead into a runaway.

The Celtics, who lost to the Golden State Warriors in the NBA Finals last year, will face the Miami Heat in the East finals for the second straight season. Game 1 is on Wednesday in Boston.

“I was relieved just to get another chance. Our season could have been over after Game 6,” said Tatum, who broke the Game 7 record of 50 points set by Golden State’s Stephen Curry two weeks ago. “It definitely was on my mind that I had played as bad as it could get, for 43 minutes.

“We had a saying: ‘It’s only up from here.’”

Tatum added 13 rebounds and Jaylen Brown scored 25 points for Boston, which rallied from a 3-2 deficit in the series to keep alive their hopes for an unprecedented 18th NBA championship.

“You always come into a series with the expectation of how it’s supposed to go,” said Celtics Coach Joe Mazzulla, who is in his first year as an NBA head coach after being promoted on the eve of training camp when Ime Udoka was suspended. “That’s not how the playoffs are.”

Tatum was subbed out of the game with three minutes left to a standing ovation from the crowd, which soon broke out in a “Beat the Heat!” chant. Miami beat Boston in six games in the 2020 East finals and the Celtics took a seven-game matchup last season.

“It’s the best time of the year,” Tatum said. “As a competitor, I love the opportunity.”

Newly crowned NBA MVP Joel Embiid scored 15 points on 5-for-18 shooting, and Tobias Harris scored 19 for Philadelphia. The Sixers lost in the conference semifinals for the third straight year, and the fifth time in six seasons; they have not gotten any farther since reaching the NBA Finals in 2001.

“I thought we had the right group. I really did,” said Sixers Coach Doc Rivers, who was on the Boston bench when the Celtics won it all in 2008. “We played great all year and this loss absolutely diminishes what we did this year in some way. … I think this team is headed right. I thought we took another step this season. And then tonight I think we took a step backward. But that’s OK. That happens, too.”

Tatum started 0 for 6 in a Game 5 loss and made only one basket in Game 6 before erupting for four 3-pointers in the final 4:14 to force the decisive seventh game.

He picked up where he left off, scoring Boston’s first basket and 11 points in the first quarter, 14 more in the second, and outscoring the Sixers on his own, 17-10, in the third. The 33-10 edge in the third was the most lopsided quarter in a Game 7 since at least 1997.

“JT just got it going and get out of that man’s way,” Brown said. “He got it rolling, and there’s nothing they could do to stop him.”

BANGED-UP BROWN

Brown began the game with the black mask he has worn since breaking a bone in his face before the All-Star break. He took it off and soon absorbed an elbow to the face from James Harden, who was assessed a flagrant foul.

Brown continued without the mask but finished the half with cotton in his left nostril. He also played with an edge, making both foul shots and following it with a steal and layup as Boston went on a 9-0 run to erase a 35-26 deficit.

Brown also dove into the Sixers bench to save a ball, then had words with the bench after Georges Niang grabbed his leg and kept him from getting back on the court. Both players were given technical fouls.

Williams followed the commotion with a layup that tied the score 35-all.

“Nothing like a shot to the face to wake you right up,” Brown said.

WHAT’S UP, DOC?

Rivers is 6-10 in seventh games. The 10 defeats are five more than any other NBA coach and three more than the NHL’s Mike Babcock and Bruce Boudreau. His teams are 6-16 in their last 22 chances to close out an opponent in a playoff series.

Asked if he expected to be back next year, Rivers said: “Yeah. I think I got two years left. No one is safe in our business and I get that.”

Harden also could be gone if the Sixers opt for an overhaul. He has the opportunity to become a free agent.

“James has a decision to make this summer and he’ll make it,” Rivers said. “We asked him to play a different way than he’s been accustomed to. And he did it.”

TIP-INS

Tatum is the sixth player since 1997 to score 25 points or more in the first half of a seventh game. The last was Luka Doncic, who had 29 against the Clippers in the first round of the 2021 playoffs. Paul Pierce scored 26 for the Celtics against Cleveland in the second round in 2008. … The 10 points Philadelphia scored in the third quarter were tied for the fewest in any Game 7 quarter in the shot-clock era.