Jordi Fernandez trying to get Ziaire Williams back to Nets potential

The Nets need Ziaire Williams to be their best defender, to get back to the way he played a year ago.

So far, they haven’t seen it this season.

Coach Jordi Fernández has tried to coax the best out of Williams, first with words and then with (lost) playing time.

Williams took his first healthy DNP of the season Tuesday vs. the Knicks, a clear message about his need to improve.

It remains to be seen if the message was received.

It won’t be sure until the Nets take the court Friday — that is, if Williams even plays vs. the 76ers.

“It’s a very abstract question because I just talked to him and didn’t play him last game,” Fernández told The Post. “So if — whenever he has the next opportunity to play — he consistently does it, then we will see if I was successful or not. If not, it’s not on him; it’s on me to try to find ways for him to perform consistently.

“Energy is not a subjective thing. … What we need is his ball pressure to be elite, his deflections to be elite, his pick-and-roll defense to be elite, his iso defense to be elite. … Last year, he was above average, one of the top players in the NBA. We need that to be the same or better because I know he can do it.”

While Williams’ 3-point shooting improved, that outside stroke was just a bonus. What the Nets needed — and got — was defense.

But after they re-signed him to a two-year, $12.5 million deal this summer, Williams has fallen off badly.

Williams’ defensive rebounding rate has cratered from an elite 94th percentile to a horrid 21st percentile, per Cleaning The Glass.

His steal percentage has dipped from the 76th percentile to a middling 60th.

And his defensive rating is a career low on both the league stats and Basketball Reference.

After subtly prodding Williams earlier in the season, Fernández took a stronger tack by pulling him from the rotation altogether this week.

“I wanted to challenge him with his defense,” Fernández said. “Last year, he was elite in a lot of the things that we care [about] defensively, from ball pressure to deflections to pick-and-roll defense to defending isolations. He was huge and I haven’t felt that energy. I can go through the numbers and they were not there. So, I challenge him to do that. I wasn’t all the way happy with the last two games.

“It’s just about giving an opportunity to somebody else, [and for Williams] to refocus, to be ready for the next opportunity. And when it comes, then just take it and sustain it and be the best defensive player on the team and one of the best in the NBA, which I believe he’s more than capable of doing.”

While Williams is leading the team in deflections again, his iso defense has fallen off a cliff.

He smothered foes to a stingy 0.79 points per possession last season and 39.5 effective field goal percentage, but has seen those balloon to 1.09 and 50 percent.

Other than rookie Drake Powell, no other Net has more perimeter defensive potential than Williams.

But will he ever reach it?

“Drake … has shown defensively the sky’s the limit for him, and with Ziaire I feel the same way,” Fernández told The Post. “We played very good games with him as our best defender, and I know he can keep doing it consistently. So, that’s what I’m looking for, just consistency on that end, because he can do it without a doubt.”

Williams, still just 24, acknowledged his struggles earlier this month.

“I need to do a better job for my teammates of holding my assignment better and being dialed in at all times. So, I feel like I’ve let them down a little bit [at times],” Williams said, admitting he’d been pressing.

“Yeah, for sure. Definitely. Just trying to do too many things at once. Trying to save the world in one play. … It’s all a learning process.”

Michael Porter Jr. is out against Philadelphia with low back tightness

Ben Saraf — who has been on G League assignment with Long Island — has been upgraded to questionable Friday, trying to return from a left ankle sprain.

Fellow rookies Nolan Traore and Danny Wolf are still on G League assignment, while Cam Thomas (hamstring) and Haywood Highsmith (knee) are still out.