“He’s obviously the best player in the game,” Philly native Marcus Morris, who during the week boasted that he was up to the challenge of guarding LeBron James. “Because I’m a competitor. He’s the best player, and I’m going to be able to tell my kids this one day.”

By Peter Gleason

Morris scored 21 points and added 10 rebounds while defending James into seven turnovers and only 15 points — and the Celtics opened a 21-point, first-quarter lead to scorch visiting Cleveland 108-83 in Game 1 of the Eastern conference semis.

Jaylen Brown scored 23 points and Al Horford had 20 for Boston, which ran off 17 points in a row in the first and never allowed the Cavaliers within single digits again. The Celtics led by 28 when Cleveland coach Tyronn Lue pulled James for good with 7:09 left.

Game 2 is Tuesday night.

“I have zero level of concern at this stage,” said James, who was 5 for 16 from the floor and missed all five 3-point attempts.

“I’ve been down before in the postseason, but for me there’s never any level of concern — no matter how bad I played tonight, with seven turnovers, how inefficient I was shooting the ball,” he said. “We have another opportunity to be better as a ball club coming in Tuesday night, and we’ll see what happens.”

Kevin Love had 17 points and eight rebounds, and James added nine assists and seven boards. The Cavaliers missed their first 14 3-point attempts of the game and shot just 32 percent in the first half.

By that time, Boston led 61-35 — the biggest halftime playoff deficit in James’ career.

“I think we’re very alert to the fact that we’ll get a heavyweight punch on Tuesday night,” Celtics coach Brad Stevens said. “It’s another great challenge, another great opportunity to experience something for this team.”

A year after dropping the first two games at home against Cleveland in a five-game East final, the Celtics pounced on James, and the favored Cavaliers were never really in it.

“The last couple of playoffs, and our meeting at the end of year, they blew us out of the water,” Boston guard Marcus Smart said. “We’ve got a different team, just like they do, and a lot of younger guys. So for them to see that, and have that feeling like we did tonight, is huge.”