The Knicks point-guard derby featuring three former lottery picks will officially commence in Monday’s preseason opener in Washington, but Dennis Smith Jr. will have to sit this one out.

Smith has a strained back, leaving coach David Fizdale to start either new-signee Elfrid Payton or emerging Frank Ntilikina.

The smart bet is Payton, 24, who signed a one-year $8 million pact, gets the call. However, the organization does need to see as much of their 2017 lottery pick as possible because of their Oct. 31 deadline to exercise Ntilikina’s fourth-year option on his rookie contract.

At Saturday’s open scrimmage at Columbia, Payton looked smooth in expertly running the show and defending. He is a former Scott Perry lottery pick in Orlando and admitted it was one reason he signed here.

“I feel comfortable — I feel comfortable with what we’re doing here on both sides of the ball,’’ Payton said. “They’ve been doing a good job emphasizing it to us. Whatever minutes I get, [I] will try to compete at a high level, play as hard as I can and run this offense as crisp as possible.”

Payton also has been impressed with Ntilikina’s game — surprised at the reputation he’s gained as an underachiever across his first two seasons.

“Frank is very talented,’’ Payton said. “I think he’s been given a bad rap as far as the league goes. He’s very talented, extremely long arms, especially on the defensive end. He had the chance to run the point [Saturday] and ran it well and throughout camp. I can learn from him, he can learn from me. It’s crazy how this league can be. I never thought of him as that. It’s crazy how this league tries to label you.”

Fizdale enjoyed Payton’s performance in Saturday’s scrimmage, but made sure to praise Ntilikina, too. His stock has risen in the Knicks’ eyes after a successful World Cup showing when he led France to the bronze, upsetting the U.S. along the way.

“I thought he and Frank both did an excellent job of organizing us, pushing the pace, defending, making plays for other people,’’ Fizdale said.

Fizdale has loved Smith’s defense in camp but back issues have plagued him. The explosive ex-Maverick lottery pick missed 10 of the final 14 games last season with a misplaced disk. Smith dropped 15 pounds partly to alleviate back stress.

“We’re going to be real cautious,’’ Fizdale said.

In Saturday’s scrimmage, Ntilikina hit a step-back 3 and took the ball to the hole, going up for a dunk on Bobby Portis. Though the ball didn’t fall, it was a striking sign of his new aggressiveness.

“I felt comfortable during that scrimmage,’’ Ntilikina said. “It was a great scrimmage for us playing with high level of intensity.”

Ntilkina admitted “little details” of his jump shot have been tweaked. “Not about my form but about the rhythm, how you prepare your shot,’’ Ntilikina said. “It was a lot of work done. Definitely, I was more comfortable shooting the ball [at the World Cup] and that was the goal.’’