The Celtics were pressed in the third quarter by the Mavericks. They responded the way championship teams do. - The Boston Globe (2024)

The Celtics seized the series opener in front of a raucous — and then briefly nervous — crowd, defeating Luka Doncic and the Mavericks, 107-89.

They did so by building a 37-20 lead after the first quarter, riding the inspired all-around performance of Kristaps Porzingis (playing his first game since April 29) to a 63-42 lead at halftime, and then withstanding a third-quarter Dallas surge that hacked the lead to 72-64.

“When they cut it to 8,” said Jaylen Brown, “that’s when the game started.”

And that’s when Brown and the Celtics provided their home crowd a much-needed dose of reassurance.

When the exasperatingly excellent Doncic drilled a 27-foot 3-pointer at 4:28 of the third quarter, cutting what was once a 29-point lead down to that tense 8-point margin, Celtics coach Joe Mazzulla called a timeout, something he may or may not have done last season.

The Celtics, who had hit just two of 10 shots and one of seven 3-point attempts in the quarter, came out of the breather composed and ready to take command.

They tore off a 14-0 run until the Mavericks’ Daniel Gafford hit a pair of free throws with 14.1 seconds left in the quarter.

Thanks in large part to Brown, who scored 6 of his team-high 22 points and blocked three shots during the run, the Celtics took an 86-66 lead into a suspense-free fourth quarter.

The Celtics played their most disciplined and determined basketball when the moment demanded it. It may have just been the third quarter, but what Brown and the Celtics did absolutely counts as closing out a game.

It was a moment of truth, and further validation — after a string of clutch performances in the Eastern Conference finals — that this team is capable of playing its best when the tension is highest.

“When a team goes on a run, you’ve got to manage it, you’ve got to stay composed, and you’ve got to keep playing basketball,” said Brown. “It’s almost like you just have short-term memory a little bit, like the team’s not even on a run. You’ve got to play smart basketball and make great plays to get flow back.”

Mazzulla said he spoke to the team for 30 seconds during the three-minute timeout, meaning the players had 2½ minutes to converse among themselves.

After the game, Mazzulla looked like he’d rather chew his microphone into bits than answer specifically about what he told his team.

And Porzingis, well, he couldn’t recall his coach’s specific words. But the tone stuck with him.

“I don’t know,” said Porzingis after a pause, “but Joe always gives us the right message, you know what I mean? He can be emotionless if that’s what we need and he can just come in, completely even keel, and give us the right message of what to focus on.

“And then he can come in and give us some motivation if that’s what we need in that moment, some energy.”

Brown did give up some detail about what the players said to each other.

“We just said, just breathe,” he said. “The game is starting now. Just breathe.

“This is a moment where our experience shines through. Just breathe, just keep playing basketball. If you’ve got a shot that’s open, take it with confidence. No turnovers, take care of the basketball and just play our game.

“We’ve got to get some stops. They made some big shots. Just navigate the run. That was it.”

The Celtics didn’t just navigate it. They seized control of the game, sailed smoothly through the final quarter, and got the start they desired in this series.

No, the performance was not flawless. Jayson Tatum hit just 6 of 16 shots — including a couple of forced 3-pointers during the Mavericks’ surge — and committed six turnovers, while Payton Pritchard was 0 for 7 off the bench.

But those are just notes for Nitpick Corner.

Tatum, who collected a game-high 11 rebounds and tied for the team lead with 5 assists, demonstrated again that he has learned to contribute in myriad ways when his shot isn’t cooperating. He is not required to be a one-man band, because the Celtics (who had five players score between 12 and 22 points) are a complete team.

If you picked the Celtics to win this series in six games — or fewer — there is no reason after Game 1 to recalibrate that expectation.

This is the best team in the NBA. They played like it in the first half of Game 1, and they played like it in those crucial final minutes of the third quarter, right when familiar doubts that originated with Celtics teams of the recent past might have begun to creep in.

And now they are three wins from their first NBA title since 2008, and the 18th in franchise history.

Not that the Celtics are counting victories before they are completed, of course. Go ahead, call that another sign of their growth.

“Every game has its own story,” said Brown. “We’ve just got to stay ready, stay composed, and take it one game at a time.”

Chad Finn can be reached at chad.finn@globe.com. Follow him @GlobeChadFinn.