BOSTON — LeBron James was on fire to begin Game 2 of the Eastern Conference finals.

The Cleveland Cavaliers superstar scored 21 first-quarter points — his career high in scoring for a quarter in the playoffs — en route to a 42-point triple-double Tuesday night at TD Garden.

Normally that kind of epic performance would be enough for the Cavs to steal a game on the road, but it wasn’t enough to beat the Boston Celtics, who overcame an 11-point first-half deficit to take a 2-0 series lead with a 107-94 win.

After a subpar performance from James in Game 1, everyone was expecting a monster bounce-back in Game 2. For the Celtics to take a heavy blow from James, keep upright and finish the job says a lot about their character and resiliency.

“He’s a great player. We came out, played some team ball and got the win,” Celtics forward Jaylen Brown said.

“When somebody comes out like that, it’s easy to get discouraged. Especially when time after time it seems like we’ve seen that over and over, LeBron just dominates the game and they win. I was just trying to relay to my team ‘keep staying the course, he’ll throw punches at you guys, just throw punches back.’”

Brown had a good first quarter of his own, leading the Celtics with 14 points in the opening 12 minutes to help the C’s go into the second quarter trailing by just four. Boston came out of halftime very aggressive and outscored Cleveland 59-39 over the last two quarters to earn a statement win.

The game plan for the Celtics all series is to make it hard on James. They know he’s going to score a lot because he’s arguably the best player of all-time, but they can’t make it easy for him. James scored 21 points over the final three quarters because Boston’s defense forced him to take much tougher shots than he was getting in the opening frame.

The Celtics again threw multiple defenders at James, led by Marcus Morris.

“Tremendous, tremendous first quarter. I think the biggest thing is we’re just trying to make it as hard as possible,” Celtics coach Brad Stevens said. “And I think that we all have a good grasp on that that’s not always going to work, so don’t hang your head, go down and score.

“And then if you can make it as hard as possible for as long as possible, maybe you can get somewhere. So we’ve got a lot of different guys guarding him. Jaylen started the game on him, Morris is guarding him a ton. I thought Semi (Ojeleye) did a good job on the couple of possessions he guarded him, which is really hard to just come off the bench not in the flow of the game and put a body in front of him, but he’s unbelievable.”

The Cavs wasting a great performance by James was a real missed opportunity.

Not only did they fail to take a game on the road, which they must do to win the series, they’ve helped give an already-confident Celtics team even more belief that winning this matchup and reaching the NBA Finals is possible.

Thumbnail photo via Greg M. Cooper/USA TODAY Sports