Kristaps Porzingis needed to gather himself a couple of times during the Knicks’ home opener. But he’s no longer a rookie, so it wasn’t that difficult.

The 21-year-old let the game come to him and didn’t force anything in the 111-104 win over the Grizzlies on Saturday night. He brought the Madison Square Garden crowd out of its seats with a third-quarter dunk, when Porzingis’ athleticism and agility were on full display.

He was ahead of the field, and when Kyle O’Quinn threw him an outlet pass, an easy breakaway dunk was coming. But when Grizzlies guard Andrew Harrison caught up, swiped at the ball and pushed Porzingis after he elevated, the 7-3 forward kept his balance in the air, stretched his long arms over the rim and dunked it.

“I wanted to go off of two legs and go strong to the basket,” Porzingis said. “I was able to get to the rim with him trying to foul me. I think the work that I put in during the summer, the leg strength that I gained, helped me in that situation.”

Jeff Hornacek said: “I don’t know why a guy would go try to block a 7-3 guy on the break like that . . . He’s a great player. He’s going to continue to get better.”

As impressive as the dunk was, Porzingis’ reaction was equally notable. Poised and mature, he didn’t bang his chest or wave his arms the way so many players do. He looked into the crowd, slapped hands with teammates and converted the three-point play.

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Porzingis led the Knicks with 21 points and shot 7-for-11 from the field.

“I wasn’t as anxious as I was last year,” he said. “I knew I had to calm myself down. I was still hyped, I was ready to go.

“At least I knew how to calm myself down. I think last year when I started, the first shot I took was an air ball. You could tell I was too excited. This year, I was excited, but I knew I had to relax a little bit and just let the game come to me. I did and it ended up good.”

Porzingis said talking to himself helped calm him. He will have room to grow with the veterans the Knicks have around him.

Derrick Rose got a lot of attention in his Knicks home debut for his ankle-breaking crossovers and drives. Joakim Noah played with passion and energy, was a facilitator with seven assists and heard his name chanted several times.

Courtney Lee bounced back from a scoreless opening night with 16 points. Carmelo Anthony scored 20. He took only two shots in the fourth quarter but scored eight points during a critical time down the stretch.

Anthony embraces not having to do all the heavy lifting anymore, and he believes it will make the Knicks better in the long run.

“Derrick had it going, Kristaps had it going, and when it came to me, it came to me,” Anthony said. “I wasn’t going to go out and try to force the issue because I didn’t have the touches that I normally get. Every game is going to be different.

“I look forward to just being out there with guys who can control the game and make plays, and when it’s my time, it’s my time.”