BOSTON -- More than 15 minutes before Game 6 of the Eastern Conference finals tipped off at TD Garden, every seat inside the building was occupied, and every person ticketed to sit in them was standing.

After 12 years of waiting, this crowd -- and this city -- was ready to return to the NBA Finals. Now, they'll have to wait another two days -- at least.

Thanks to an all-time classic performance from Jimmy Butler, the Boston Celtics saw their dreams of a trip to the league's championship round vanish, falling 111-103 to the Miami Heat to send this series back to the shores of Biscayne Bay for a seventh and deciding game Sunday night.

"It's a tough one," Celtics guard Derrick White said.

That is an understatement.

Boston spent the two days between its Game 5 victory in Miami and Game 6 tipoff talking about the need to come out and play this game like it was a Game 7. Instead, the Celtics came out and -- like they have so many times both in this series and in these playoffs -- carelessly turned the ball over time and time again, racking up several in the game's opening minutes.

So, rather than allowing the crowd, which was desperate to get into this game, to take over, Boston gave Miami life. And, for a team that was seeking any reason to have hope after a rough second half in Game 5, it quickly ignited into a full-fledged flame, as Miami jumped out to a 29-22 advantage after one and held the lead for the vast majority of the game.

And, when Boston finally leaped back in front midway through the fourth on a 3-pointer by White -- who had 22 points off the bench and, at one point, appeared he could be headed into the annals of Celtics history -- with 4 minutes, 43 seconds to go, the Heat responded with a 17-6 run to end the game and send this series back to Miami.

"It's kind of indicative of how our nights have been in this series when we don't take care of the ball," Celtics coach Ime Udoka said of Boston's slow start. "A lot of careless ones, unforced, and that got us behind.

"Throughout the game any time we got within striking range, it felt like we had a poor decision, and they got out and scored.

"[We] had chances and didn't take advantage of them."

While Boston didn't take advantage of its chances, Butler more than did so with his. In a performance that was eerily reminiscent of the one LeBron James had in this building, in this exact same situation and for the same team 10 years ago, Butler finished with 47 points, 9 rebounds and 8 assists, hitting one difficult shot after another as he put the Heat on his back and carried them over the finish line.

"He just had it rolling," Jaylen Brown said. "Made a lot of shots that, on film, we'll live with. Made 4 3s, I don't think he's made 4 3s in a game all playoffs. But tonight he came out and he was aggressive.

"He played like his back was on the wall and he had an amazing game, and we just had no answers for him tonight. We've got to do a better job ... we will do a better job on Sunday."

While Butler was torching the Celtics, Boston couldn't get either Brown or Jayson Tatum going. While Tatum finished with 30 points, and Brown 20, they each had 18 in the first half.

In the second, they combined for 14 points on only seven shot attempts -- including one each in the fourth quarter.

"I think it's just the flow of the game, how the game was going," Tatum said of his lack of shots in the second half. "Obviously, I've got to watch the film and things like that, but I think being out there and just the feel of the game, drawing a lot of attention, trying to find mismatches.

"I think it was just kind of how the flow of the game was going."

The flow of the game was going against the Celtics seemingly the entire time, thanks to an iconic performance from Butler, a gutty one from Kyle Lowry, who had 18 points and 10 assists while playing on a bad hamstring, and the Celtics once again failing to get out of their own way.

That has now put them in a position where their season will come down to a Game 7 on the road, where Boston will either punch its ticket to the NBA Finals for the first time in 12 years, or will spend the entire offseason -- and maybe longer -- thinking about what could have been.

Despite the setback Friday, however, Tatum remains confident in his team, and didn't hesitate when asked about his belief in Boston's ability to win Game 7.

"Scale of 1 to 10?" Tatum asked with a smile. "Ten.

"I mean, it shouldn't be any less than that, right? You know, it's the last game. That's what it's all about. It's a 10 out of 10 in my confidence level and the group."

The Celtics spent the past two days saying they needed to treat this game like a Game 7, in order to avoid having to put themselves in a position to do what they must now. They talked about how Boston did just this very thing against the Milwaukee Bucks, winning Game 6 on the road after being down 3-2 before coming home and taking care of business in Game 7.

Now, the Celtics will spend another two days wondering if failing to take advantage of this opportunity has written a far different ending than this team would have hoped.

"We won the close-out game, obviously, [against Milwaukee], and for us we've won two in Miami, so that's the positive of it," Udoka said. "We know we can go there and win.

"But just have to make it harder than it is, and at times it feels like we're doing that, not taking advantage of what's in front of us, and just overall sloppy basketball on both ends. We can't have that on the road."