Ex-unified heavyweight champion Andy Ruiz Jr has the heart and quality to reign again according to one of his former opponents.

It is exactly a year since Ruiz and his deceptively fast hands battered Anthony Joshua to a seventh-round loss at Madison Square Garden – an outcome that was instantly ranked alongside the biggest upsets in boxing history.

However, the 30-year-old's stint as Mexico's first heavyweight champion proved short-lived, as Joshua took the IBF, WBA and WBO belts back into his possession with a lopsided points win in Saudi Arabia last December.

In the aftermath, Ruiz's focus was questioned on account of his rotund appearance. He weighed 15 lbs more than in the original encounter to top 20 stone and turned in a ineffective performance against a newly streamlined Joshua.

Indulging in the bounty from his shock triumph over Joshua is something familiar to heavyweight veteran Kevin Johnson.

Now 40, Johnson was undefeated with 22 wins and a draw from 23 career contests heading into a unanimous points loss against Vitali Klitschko in 2009.

Although he responded by winning his next six bouts, the American never returned to world title contention and is now a respected journeyman opponent – having shared rings with the likes of Joshua, Tyson Fury, Dereck Chisora, Kubrat Pulev and Ruiz himself.

Speaking to Stats Perform News, Johnson insisted boxing's big men had not heard the last of Ruiz, who he lost to over 10 rounds in 2018.

"Andy Ruiz is going to conquer again in the division. You can't sleep on him," he said.

"He made the same mistake I made after I fought Klitschko. You can't make all that money and then go and do whatever you want to do. You've got to stay focused.

"There was a rematch to come, he didn't stay focused and that's what happens. He got way more money than I got, so I can imagine!

"I know Andy, I know his pops. I love those guys. I've been out in LA with him one-on-one.

"He's one of the greatest fighters that you can ever sit down and talk to, you wouldn't even think he was a fighter. It will fool you.

"He has the greatest spirit of all fighters I know in the world."

Ruiz has frequently posted training clips on social media over recent weeks as he awaits a return to competitive action, having joined up with Saul 'Canelo' Alvarez's trainer Eddy Reynoso in the aftermath of the Joshua loss.

Johnson knows first hand that natural gifts remain – ones often overlooked when it comes to a boxer who is far from body beautiful.

"When I fought Ruiz the first four rounds were hell because I did what AJ did," he recalled.

"You can't stand and trade with a guy who's a sniper. He's fast, very fast. Don't let the weight fool you.

"Under all of that fat is a great conditioned guy – great! And fast."

As Joshua, Fury, Ruiz and the other leading lights in the heavyweight division wait for boxing in the UK and America to plot its return amid the ongoing coronavirus pandemic, Johnson is in the unique position of having a fight date confirmed and ready to go.

He will face fellow former world title challenger Mariusz Wach at Palac w Konarach in Konary, Poland, as part of a behind-closed-doors event on June 12.

"It's going to be under a strategic, surgical eye as far as the methods and precautions are concerned," said Johnson, who has undertaken his preparations in Gelsenkirchen, Germany with trainer Christian Hiller.

"It's going to be very different to any show that's been done because of the extremes we have to go to for our safety.

"The government in Poland have approved a very strict venue. Everyone's going to get tested, even the camera crew, the referees, the judges. Everyone."