The Celtics entered the NBA playoffs as the odds-on favorite to win the Eastern Conference.

They looked the part Sunday afternoon in their postseason opener.

No. 2 seed Boston steamrolled the seventh-seeded Philadelphia 76ers, 123-91, at TD Garden to take Game 1 of their first-round series. Game 2 of the best-of-seven is set for 7 p.m. on Tuesday.

Jayson Tatum looked sharp in his first playoff outing since he ruptured his Achilles in last year’s East semifinals, finishing with 25 points on 9-of-17 shooting, 11 rebounds, seven assists, two steals and one turnover. Jaylen Brown shook off a slow shooting start to lead all scorers with 26 points on 11-of-21 shooting.

“These last 48 weeks (since surgery), it’s kind of been a buildup to today,” said Tatum, who scored 21 of his points before halftime.

Boston’s lead hit 30 early in the fourth quarter, allowing head coach Joe Mazzulla to rest his top stars as the team begins what it hopes will be a months-long playoff run. Tatum played 32 minutes in the win; Brown played 30.

Defensively, the Celtics put the clamps on Tyrese Maxey, holding the NBA’s fifth-leading scorer to 21 points on 8-of-20 shooting. Limiting Maxey’s transition opportunities and keeping him off the foul line were points of emphasis for Boston entering the series, and he attempted just five free throws in 37 minutes.

Without former NBA MVP center Joel Embiid, who underwent an emergency appendectomy on April 16 and has yet to return, the Sixers’ supporting cast couldn’t make up for Maxey’s inefficiency. Philadelphia shot just 38.9% from the field and 4-for-24 from 3-point range.

Boston outscored Philly 48-12 on 3-pointers, 20-10 in transition and 22-3 in points off turnovers. The 32-point margin of victory was the largest in a Game 1 in franchise history.

“That was Celtics basketball,” Brown said. “We’ve been the harder-playing team all year. That can’t change now that the playoffs have started.”

Sam Hauser (12 points, seven rebounds), who came in shooting 52.6% from deep in April, sparked the Celtics’ offense with two early threes. He also drew a charge against Adem Bona, sending the Sixers’ fill-in starting center to the bench with two fouls just 97 seconds after tipoff. It was Hauser’s first drawn charge of the season.

Celtics center Neemias Queta slammed home a driving one-handed dunk on the next possession. But he also picked up two quick fouls, leading to early minutes for backup Nikola Vucevic.

Hauser’s second 3-pointer kicked off a 14-2 run that put the Celtics ahead 22-9. Brown, who struggled as a shooter early in the game, forced turnovers on consecutive Sixers possessions during that stretch, first drawing an offensive foul on Andre Drummond and then intercepting a Paul George pass. The latter led to a fast-break layup by Tatum — one of nine baskets the Celtics star made or assisted on during a sensational first quarter.

Tatum’s stat line after one: 12 minutes, 10 points, 5-for-7, seven rebounds, four assists, one steal, one turnover. Boston ended the quarter with a 33-18 lead after Tatum slithered through three Sixers defenders for a dunk, then shot an outlet pass to Jordan Walsh for a transition layup.

Mazzulla inserted Luka Garza and Baylor Scheierman to start the second quarter — the ninth and 10th Celtics to see action in the opening 13 minutes. Mazzulla typically does not go that deep into his bench during the postseason — he’s fielded an eight-man rotation in most of his playoff games as head coach, including nine of the 11 last year — but both players made the most of their minutes.

Scheierman hit a floater and blocked a George layup. Garza drew a foul on an offensive rebound and canned a 3-pointer, though he also was called for two shooting fouls in his first three-minute shift. (Fouls were an issue for big men on both sides; the five who played were whistled for 13 in the first half alone, plus a technical foul on Philadelphia’s Andre Drummond.)

“We play our rotation,” Mazzulla said. “We play our guys. It’s what we’ve done the entire year. It’s what we’ll continue to do.”

By the time Tatum checked back in with 7:16 to go before halftime, the Celtics had extended their lead to 49-29. He then scored 11 of Boston’s final 13 points of the half — six of them on free throws — to send the C’s into the locker room with a 64-46 advantage. Tatum topped 20 points in the first half of a playoff game for the 16th time in his career.

“Defensively, I thought he was really good,” Mazzulla said. “Rebounding was great. He changed matchups six, seven, eight times throughout the game. I thought that was great there. Offensively, he just gave the game what it needed. He was seeing aggressive pick-and-roll coverage. With that, you have to be able to make the right play for yourself and others. I thought he did a great job of that, gave the game what it needed. We’ve got to continue to do that.”

The Celtics built that lead by feasting on Sixers miscues while mitigating their own. They scored 13 points off turnovers to Philly’s one in the first half. Boston also finished effectively at the rim (8-for-10) and was efficient on non-paint twos (5-for-8), with Tatum excelling in both areas.

Philadelphia showed signs early in the second half, but Brown wrestled back momentum by stealing a Drummond outlet pass and drilling a 3-pointer. Vucevic hit another on the next Celtics possession — his first made field goal in the postseason since 2022 — followed by a Brown dunk and a Tatum layup. The 10-0 blitz made it 83-58.

Brown scored 16 points in the third quarter, then sat for the rest of the game. Tatum played the first five minutes of the fourth quarter before checking out with Boston ahead 107-78.

As both teams began emptying their benches, the Garden crowd chanted “We want Boston,” mocking the chant that broke out at the 76ers’ home arena during their play-in victory over Orlando.

“We’re prepared,” Brown said. “We were ready. So we’ve just got to be ready every time we step out on the floor. But (this was a) great tone-setter. … It sets the tone and it reflects our focus and our intensity.”