BOSTON — With 6:27 remaining in just the second quarter of Game 1 of the playoffs, Al Horford couldn’t hide his emotions.

The Celtics were up 22, and the Hawks earned second- and third-chance opportunities. But Horford wouldn’t allow there to be a fourth. The big man corralled a rebound over Atlanta’s Onyeka Okongwu, drew a foul and then continued dribbling down the court. He took two dribbles before taking a few skips, clenching his right fist and letting out a few screams before slapping the ball.

It was a rare display of emotion for Horford, the 36-year-old steady, mild-mannered veteran who picks his spots to do so. Even though it was only Game 1 of the first round, it was the strong opening statement he and the Celtics were looking for as they restart their pursuit of a championship that they came so close to claiming a year ago.

Horford’s expressions fired up the TD Garden crowd and spurred the Celtics to a 30-point halftime lead. Then, when things got dicey in the fourth quarter, it was Horford’s voice that helped rally the C’s, who had one of their best defensive performances of the season and delivered a strong first punch in their 112-99 victory over the Hawks.

“It’s a great time of the year,” Jaylen Brown said. “It’s the time we all have been thinking about, waiting around for so it was extremely important to set the tone tonight here on our home floor to open up the playoffs.”

The Celtics had been waiting for this moment all season, and especially the last few weeks. Jayson Tatum, earlier in the week, admitted the Celtics started looking forward to the postseason over the final games of the season as the postseason and another crack at a championship loomed.

It showed. On the first play of the game, Brown blew up a play on defense, then scored the first points of the game. That fueled the Celtics to one of their most dominant first-half efforts of the season. They were locked in defensively behind Marcus Smart and Robert Williams. They limited Trae Young to 16 points on 5-for-18 shooting, representative of a difficult day for a Hawks offense that finished just 5 for 28 from downtown.

Offensively, Tatum (25 points, 11 rebounds), Brown (game-high 29 points) and Co. tore apart the Hawks’ lackluster interior defense to get anything they wanted as they scored 74 first-half points, which was enough for the inevitable coast down the stretch.

It looked like the Celtics would run away in a blowout, but their focus and execution waned. They got careless with the basketball, ultimately finishing with 17 turnovers, and it cost them as the Hawks cut the 30-point lead to 12 twice in the fourth quarter, which made for some brief concerning moments.

“That’s just part of it,” Celtics coach Joe Mazzulla said. “I just thought we lost our offensive purpose, playing passive offense instead of just keeping it moving, being aggressive and maintaining that level of pace that we had in the first half.”

The Hawks trimmed the deficit to 12 with 9:33 to go when Horford lost the ball to Bogdan Bogdanovic, who scored on a three-point play. Mazzulla called timeout and Horford was animated in the huddle.

“Just to stay solid,” Smart said of the message. “We’re up obviously but this is where we gotta work and continue to get better. This is the time where we can’t have any slip-ups. We don’t want a game like that where they come in and cut a 30-point lead down to 12 and come back to haunt us.”

It didn’t. The Celtics answered with a 6-0 run, including Brown’s second-chance put-back that restored their lead to 18. The Hawks cut it to 12 again with 3:16 remaining but Horford responded with a 3-pointer and the Celtics — after building a huge lead — ultimately did plenty to take Game 1.

The Celtics have been known to let go of the rope at times this season, and while the game was never truly in jeopardy, there were some familiar themes and carryover in Game 1.

“You get up 30 and kind of just relax a little bit,” Derrick White said. “Whether it’s the playoffs or not, we’ve got to be better with that. We can’t take our foot off the gas and kind of slow down like we did in the second half. But a lot of stuff we can improve on, make adjustments on, and get better about.”

The Celtics know they can be much better. Mazzulla jokingly said he was concerned about it but thinks there’s a benefit to their miscues. The C’s won the rebounding edge, but still gave up 14 offensive boards, which is a major point of emphasis in this series. They had 16 turnovers, which was a major flaw of last year’s postseason run and something they know they need to clean up.

“That’s going to happen,” Mazzulla said. “It’s actually good that we went through it, we had to feel that, and it’s important that we learn from it. So for us to be able to get a win and at the same time know we can play better is good. …

“It’s good that you can fortunately win and at the same time, have the humility to know that you can be a lot better and we can do things better. So they got more offensive rebounds than us, we turned it over more, there’s a lot of stuff there that we can just build on. So we are fortunate in that category, and we have to just have the mindset of, ‘How can we be better?’ ”

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