LAKE BUENA VISTA, Fla. -- After the Boston Celtics walked into the locker room at halftime of Game 5 of the NBA's Eastern Conference finals against the Miami Heat, having produced a second consecutive lackluster first half in a series they trailed 3-1, they knew something had to change.

Before they returned to the court for Friday's second half, point guard Kemba Walker had a simple message for his team.

"What I remember from halftime is Kemba saying, 'We just need to settle down a little bit,'" Jaylen Brown said. "We just had to dial in a little bit, and once we did, I felt like we were fine."

It was a directive that Boston took to heart. As a result, the Celtics have basketball left to play.

A 41-point explosion in the third quarter, including 17 points from Jayson Tatum, turned on its head a game that Miami firmly controlled and allowed Boston to surge to a 121-108 victory. The series resumes Sunday with Game 6.

Anyone who watched Boston slog through the first half might've doubted that such a turnaround was possible. Like in Game 4, the Celtics seemed to be sleepwalking much of the time, allowing the Heat to gobble up several loose balls and settling for far too many jumpers instead of aggressively attacking the paint.

That changed, however, in the second half. Boston came out swinging in the third quarter and looked like a completely different team, at one point going on a 20-3 run to turn a 60-51 deficit into a 71-63 lead. The Celtics didn't relinquish the advantage the rest of the way.

"I just thought we played with great tenacity defensively, and our offense followed suit," Celtics coach Brad Stevens said. "But [the Heat are] very hard. ... Like, it's easy for me to sit up here to say to be at our very best and get stops on every possession.

"This is a heck of an offensive team, a heckuva well-coached team and hard to guard."

The Celtics made the Heat look easy to guard in this one. Miami shot 7-for-36 from 3-point range, including 4-for-26 by its starters. Boston swarmed Miami repeatedly in the second half and converted that aggressiveness into action at the other end, relentlessly attacking the rim and either scoring or getting to the foul line.

It was a combination that prompted Stevens to say during a "wired" segment on ESPN's broadcast of the game that it was "in all sincerity, the first time I've seen Celtics basketball in the last few games."

What changed?

"Everybody was so anxious, eager to make a play, make something happen," Tatum said. "We know what's at stake: We lose and go home.

"But at the same time, we've got to relax a little bit. Take a deep breath. We know how important every possession is, but we've still got to just relax a little bit and play the game, and that was kind of the message at halftime."

It was one Tatum embodied as he came out firing in the third quarter, with his 17 points nearly matching Miami's 25 in the frame as he set the tone for Boston's turnaround. Tatum told ESPN's Rachel Nichols after the game that he had trouble sleeping the past few nights before taking the court for Game 5.

Tatum, who finished with 31 points, 10 rebounds and 6 assists in 43 minutes, said his restlessness came from wanting to get back onto the court after his uneven Game 4 in which he went scoreless in the first half before contributing 28 to a second-half Boston comeback that, unlike in Game 5, fell short.

"I mean, we were down 3-1," Tatum said. "Frustrated. Give them credit: They've been playing well. They deserved to be up 3-1. It was frustrating. Not supposed to be feeling good about being down 3-1. I was just really anxious to play, get back out there, just give [ourselves] a chance."

Boston was able to give itself a chance and keep its season alive because of a group effort in that second half. Brown had 28 points and eight rebounds. Daniel Theis and Enes Kanter combined for 23 points and 17 rebounds in 45 minutes Friday and outproduced Heat star Bam Adebayo in the process.

More than anything, it was a renewed commitment to energy and effort -- as well as that friendly reminder from Walker to relax and let the game come to them -- that breathed fresh life into the Celtics' season.

"We were playing a little bit fast, a little bit antsy," Brown said. "We were trying to win the game in the first half. And we just needed to stay with it, keep making the right play and just settle down a little bit. When we did, the shots started going in. Our defensive intensity was good. We gave up less baskets in the third quarter. And we looked like the team we all know and love."