"He was absolutely fearless tonight, and it was contagious," Heat coach Erik Spoelstra said. "The way he approached the last 48 hours, and not only LeBron, but everybody else. Nobody likes getting dirt thrown on your face before you're even dead. He showed great resolve."
James shot 19 of 26 from the field and finished four points shy of his playoff career-high while playing 45 minutes, not sitting down until the victory was long secured.
After two days of questions about the Heat's future and his own history, James provided his response in resounding fashion in a building where Miami had lost 15 of its previous 16 games.
"In an environment like this, you want to have a big game," he said. "I wanted to be there for my teammates, no matter what was going on throughout the course of the game.
"This was a gut check for us, and it's good to see we were able to bounce back after that loss, after that Game 5 loss at home."
Dwyane Wade added 17 points for the Heat, who need a victory at home Saturday night to return to the NBA finals. And if James plays like this again, Miami should have no problem getting it.
"He played amazing. He was locked in from the beginning of the game like I've never seen him before," Wade said.
Rajon Rondo had 21 points and 10 assists for Boston. Kevin Garnett and Brandon Bass each scored 12 points, but Paul Pierce had only nine on 4-of-18 shooting.
In the site of some of James' biggest disappointments, the only disappointed ones Thursday were the thousands of fans who hoped to see a celebration but instead filed out of the TD Garden midway through the fourth quarter, just before the league MVP called it a night.
"He was comfortable all night," Rondo said. "We didn't get into his air space."
Miami barely won Game 2 before dropping the next three games, but this one was never in doubt. James was a one-man force on what's supposed to be a Big Three, carrying the Heat in the first half while Wade made only 1 of 6 shots.
The Celtics were hoping to complete a comeback from a 2-0 deficit, as Oklahoma City did in the Western Conference, and advance to face the Thunder in the NBA finals. But they missed 13 of 14 3-point attempts and will have to win a second straight game in Miami to play for the title.
James' season was twice ended in Boston while playing for Cleveland, the Celtics emerging as the winner in a Pierce-James duel in Game 7 of the 2008 East semifinals, then beating the Cavs in Game 6 of the second round two years later.
That was James' last game with Cleveland, leaving that summer for Miami and the All-Star help that Wade and Chris Bosh could provide.
He needed none of it Thursday.
He had 30 points by halftime and spent the night silencing the Celtics crowd and perhaps some of the doubters he somehow still has. He set the tone for the game by making 6 of 7 shots and scoring 14 points in the first quarter, and he made sure the Heat were never challenged from there.
"I hope now you guys can stop talking about LeBron and he doesn't play in big games," Celtics coach Doc Rivers said. "He was pretty good tonight. So we can put that to bed and go play Game 7."
Meanwhile, Pierce, whose 3-pointer over James was the biggest basket of Boston's Game 5 victory, missed 13 of 16 shots through three quarters, including all six 3-point attempts.
The Heat obviously weren't deflated by that loss and came to fight, scoring 10 straight points to build a 10-point lead shortly after Mario Chalmers was called for a technical foul after getting mixed up with Ray Allen. They extended it to 12 points later in the period, taking a 26-16 lead into the second after shooting 58 percent in the period.
Wade finally got on the board with a jumper to open the second, but he seemed strangely passive even with Miami trying to save its season.
No matter. James didn't need the help.
He had consecutive baskets for a 15-point lead and came up with a basket every time the Celtics tried to get any rhythm. He soared high above the rim and any other player for a follow dunk after the Celtics had crept within eight in the final 2 minutes of the half. Miami led 55-42 at the break.
James made 12 of his first 13 shots before missing from the perimeter on his final attempt of the half.
Only Rondo kept it from being a blowout, scoring 19 points and adding five assists in the half. Relishing the challenge, he even emulated Garnett by doing a couple of knuckle pushups after being knocked to the court while drawing a foul, but he couldn't keep up with James and needed some help that never arrived.
The Celtics, trying desperately for one last championship before the expected breakup of their Big Three, hope this one doesn't get added to recent series losses against Orlando in 2009 and the Lakers in the NBA finals in 2010, when they lost Game 6s with chances to win series and then fell in Game 7.
James' first basket of the third quarter increased the lead to 17 points, and from there it was just a matter of coming up with another score any time the Celtics tried to make a run.
Boston never came close, and when a 3-pointer by Shane Battier made it 81-63 with 10:19 to play, Rivers spent part of a timeout standing alone on the court with his arms folded, leaving it to the players to try to come up with an answer that wasn't there.
Bosh entered with 5½ minutes left in the first quarter of his second game back after missing nine with a lower abdominal strain, finishing with seven points and six rebounds.
Storylines LeBron, Heat force Game 7
R LeBron James totals 45 points, 15 rebounds and five assists as Miami avoids elimination.
• His Celtics counterpart, Paul Pierce, totals just nine points on 4-for-18 shooting.
• Game 7 is Saturday in Miami.
Eastern Conference finals
Game 1 • Miami 93, Boston 79
Game 2 • Miami 115, Boston 111, OT
Game 3 • Boston 101, Miami 91
Game 4 • Boston 93, Miami 91, OT
Game 5 • Boston 94, Miami 90
Game 6 • Miami 98, Boston 79
Saturday • Boston at Miami, 6:30 p.m., ESPN