Oh, You Thought the Miami Heat Were Done? Jimmy Butler Said No.

BOSTON — In a playoff series that had long ago lost any semblance of order or predictability, Jimmy Butler of the Miami Heat on Friday night emerged as a rare source of stability, and perhaps the only one.

He rose over flat-footed defenders for 3-pointers. He negotiated rush-hour traffic for layups. He drew fouls and whipped passes to teammates and left the Celtics and their fans in a state of despondence.

When so much else felt uncertain, Butler was a sure thing. It was the shared feeling among everyone in the building, for better or worse. By the time he cradled the basketball outside the 3-point line late in the fourth quarter, taking a half-beat to survey the landscape before him, he carried himself with a certain air of inevitability: Was there any doubt what would happen next?

The Celtics, so celebrated for their defense, made it easy for him. They mishandled the assignment, leaving Butler with a clear path to the hoop, and he pounced, driving for a layup and absorbing contact for good measure. It was a winning play that broke a tie game, along with the Celtics’ resolve.

“His competitive will is as high as anybody that has played this game,” Heat Coach Erik Spoelstra said.

In steering the Heat to a 111-103 victory over the Celtics in Game 6 of the Eastern Conference finals, Butler ensured that the series would be pushed to its absolute limit: Game 7 is Sunday night in Miami.

Butler collected 47 points, 9 rebounds and 8 assists while shooting 16 of 29 from the field and 4 of 8 from 3-point range. He did so on an ailing right knee after two of the roughest games of his career. He said he had been uplifted by a pregame phone call from Dwyane Wade, the former Heat guard.

“D-Wade never hits me until his voice is really, really needed,” Butler said. “And it was.”

Butler also had a one-sided conversation before the game with P.J. Tucker and Markieff Morris, two of his teammates. Tucker and Morris had a request for Butler: “Yo, we need 50.”

“He looked at us, didn’t say a word,” Tucker recalled. “He just nodded his head, kept going. I was like, oh, yeah, he’s about to play. He’s locked in.”

Spoelstra described “Game 7” as the two best words in professional sports, and he would not get an argument from the Golden State Warriors, who are awaiting the winner in the N.B.A. finals, starting Thursday in San Francisco. While Boston and Miami continue to bludgeon each other, Golden State needed just five games to eliminate the Dallas Mavericks in the Western Conference finals.

“Rest, ice, massage — all of that good stuff,” Butler said when he was asked how he would tend to his knee ahead of Game 7. “The same thing every single day.”

The Heat were coming off two straight disheartening performances. They had lost Game 4 by 20 while shooting 33.3 percent from the field. They had lost Game 5 by 13 points while shooting 31.9 percent — at home, no less, where their fans shuffled out of the arena wondering whether they would see the team again this season. After all, Butler had shot a combined 7 of 32 in those two duds while laboring with his injured knee.

In the immediate wake of Game 5, though, with the Heat facing elimination, Spoelstra did something interesting at his news conference: He channeled his inner Mister Rogers.

“You’ve got to enjoy this,” he said. “You do. If you want to break through and punch a ticket to the finals, you’re going to have to do some ridiculously tough stuff.”

He added: “We’re still alive. We have an opportunity to play in front of a great crowd, and an opportunity to make a memory that you’ll remember for a long time. That’s all we’re thinking about right now.”

Spoelstra would know, having coached the Heat to two titles and five finals appearances. In his 14th season, he acutely understands the playoffs and the stakes and the pressures and the possibilities.

If Spoelstra delivered the same message about opportunity to his players before Game 6, Butler must have absorbed every word of it before using it as fuel against the Celtics.

“His aggression just opens everything up for everybody else,” Tucker said.

In the first quarter alone, Butler shot 6 of 10 from the field and made both of his 3-point attempts while collecting 14 points, 5 rebounds and 4 assists. As a team, the Heat made five 3-pointers in the first quarter, which was especially impressive considering they had gone 7 of 45 from 3-point range in Game 5.

“I think we played with a little bit more confidence,” said Kyle Lowry, who had 18 points and 10 assists in the win. “We played with some oomph tonight, and it felt good to do it.”

While Butler’s late-game layup gave Miami the lead for good, he sealed the win with less than a minute left when he took a spinning, turnaround jump shot from 20 feet with the shot clock set to expire.

His performance as a whole evoked memories of 2012, when LeBron James scored 45 points to lead the visiting Heat to a Game 6 win over the Celtics in the Eastern Conference finals. The Heat proceeded to win Game 7 to advance to the finals, then won it all. Spoelstra declined to make any comparisons.

“It’s a different era,” he said. “It’s a different team.”

And Butler, still in search of his first championship, seems determined to make his own mark. At his news conference, he shared the dais with Lowry, who offered up a quizzical expression when Butler said he had played a “decent” game. Lowry was asked to elaborate on Butler’s game.

“It’s incredible,” said Lowry, who supplemented his assessment with an expletive. “My bad. Don’t fine me, N.B.A. That was a mistake, I promise.”

It was among the only mistakes the Heat made all night.