After the miracle and the madness, Gabe Vincent broke the silence inside the Miami Heat locker room on Saturday night by humming along to “Life Goes On,” a ballad by Ed Sheeran featuring Luke Combs.

Most of Vincent’s teammates were long gone by then, bound for their Miami-area homes as they faced the collective challenge of figuring out how to rebound from a soul-crushing loss to the Boston Celtics in Game 6 of the Eastern Conference finals.

But back in the locker room, where an oversize image of the N.B.A.’s Larry O’Brien championship trophy is stitched into the carpet and a series of murals depicting the franchise’s past triumphs line a tunnel leading to the court, the atmosphere was gloomy. The lyrics of a song about heartbreak hardly helped. They seeped from Vincent’s iPhone all tinny and hollow, as if the music were being piped through a radiator:

It hit like a train, I ran out of words;
I got nothing to say, everything hurts.

“Great song,” Vincent said.

Nothing about this season has been easy for the Heat, and Vincent hinted that perhaps some poetic justice was at work after the Celtics’ 104-103 victory in Game 6, tying the series at three games apiece. Derrick White’s astonishing putback at the buzzer — the ball left his fingertips with about one-tenth of a second to spare — had extended the best-of-seven series and the Celtics’ season, forcing a Game 7 in Boston on Monday night.

The Heat could not have been closer to securing a spot in the N.B.A. finals against the Denver Nuggets. And then, in an instant, that dream somehow felt very far away.

“It’s almost like it’s supposed to be this way,” Vincent said. “But, you know, go to Boston and get a win.”

Vincent, the team’s starting point guard, made it sound simple, but this series has been a carnival ride. The Heat won the first three games to put themselves on the cusp of history as they attempted to become just the second No. 8 seed to advance to the N.B.A. finals, joining the 1998-99 Knicks. Now, the Celtics are bidding to become the first team to win an N.B.A. playoff series after trailing by three games to none.

“This is one hell of a series,” Heat Coach Erik Spoelstra said. “At this time right now, I don’t know how we are going to get this done, but we are going up there to get it done.”

It was a public vote of confidence after a game full of missed opportunities for the Heat. Where to begin? Jimmy Butler and Bam Adebayo, their two best players, combined to shoot 9 of 37 from the field.

Butler, in particular, looked downright passive for much of the game. There he was in the second quarter, handling the ball at the top of the perimeter with the shot clock winding down. But instead of driving, Butler shoved a pass to Duncan Robinson, who had little choice but to hoist a runner from 11 feet that grazed the front of the rim. A few seconds later, the Celtics’ Jayson Tatum was at the other end for a layup.

But other plays could haunt the Heat, too. In the fourth quarter, for example, Adebayo grabbed the rim blocking a shot, which was against the rules and led to a 4-point possession for the Celtics.

As a team, the Heat shot 35.5 percent from the field. They missed hook shots and layups, jumpers and floaters. They still had a chance thanks to Caleb Martin, who slid into the starting lineup and scored 21 points, and Butler, who asserted himself late and was fouled attempting a 3-pointer with 3 seconds remaining. He made all three free throws for a 1-point lead.

But all that was prologue to the final sequence — a 3-point attempt by the Celtics’ Marcus Smart that rimmed in and out, and White’s putback. The Heat’s Max Strus had been hedging on Tatum, preventing him from getting the ball, but that left White with an open lane to the basket for the follow.

“I thought we had a lot of things covered on that play,” Spoelstra said, “and sometimes things just don’t break your way. I don’t think there’s any regrets on that. It’s just a shame.”

Butler, who scored 15 of his 24 points in the fourth quarter, shouldered the blame.

“If I play better, we’re not even in this position,” he said. “And I will be better. That’s what makes me smile, because those guys follow my lead. So when I’m playing better, I think we’re playing better as a whole.”

After finishing the regular season with a 44-38 record, the Heat landed in the play-in tournament and lost their opening game to the Atlanta Hawks. The Heat then trailed the Chicago Bulls by as many as 6 points in the fourth quarter of an elimination game before they went on a game-winning run to narrowly slip into the playoffs.

But something odd was beginning to percolate inside the Heat: The greater the challenge, the better they played. Facing the top-seeded Bucks in the first round, Miami lost two rotation players, Tyler Herro and Victor Oladipo, to long-term injuries, which should have been problematic. But Butler was brilliant as the Heat advanced in five games.

But that version of Butler has been missing as the Heat’s three-game series lead has slipped away. He has been passing up shots, hesitating on drives and turning the ball over. In other words, he looks tired from the grind of a long season.

Now, Miami is facing its greatest test yet. Butler said he planned to decompress by playing a late-night game of Spades.

“I’m not going to let our guys quit,” he said. “I don’t care what nobody says. Everything going to be OK.”

For the Celtics, the No. 2 seed, Game 7 is one more chance for them to salvage their season and make good on their pledge to return to the N.B.A. finals, one year after losing to the Golden State Warriors. Tatum has been inconsistent, even in victory, routinely going scoreless for long stretches with his season in jeopardy every night. He scored just 6 of his 31 points in the second half of Saturday’s game.

“We’re all aware it’s not time to celebrate,” Tatum said. “We didn’t accomplish anything.”

It was approaching midnight when Butler called guard Kyle Lowry to his locker for a quiet chat. Vincent had vacated the premises, taking his moody music with him.

At the front of the room, a monitor had exactly one item listed on the team’s schedule for Sunday: a 1:30 p.m. flight to Boston.