The last man to cause a seismic shift in world boxing by toppling a British heavyweight icon has warned Andy Ruiz Jr that victory in his initial spring fight with Anthony Joshua was actually the easy part.

April brought up 18 years since 20-1 underdog Hasim Rahman caused one of the biggest upsets in boxing history when he knocked out reigning world heavyweight champion Lennox Lewis in the fifth round of their infamous encounter in Brakpan, on the eastern outskirts of Johannesburg.

It was a win which etched the Baltimore man's name into the sport's history and it has since been used as the example of what can happen when a champion takes their eye off the ball for even one fight.

Lewis, it was said, was already looking ahead to a potential superfight with Mike Tyson and his training camp ahead of facing Rahman had been interrupted by filming a cameo appearance in Ocean's Eleven.

He paid the price that night in South Africa as Rahman relieved him of his WBC and IBF heavyweight titles but he got his revenge within seven months at the Mandalay Bay in Las Vegas.

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A rejuvenated, refocused Lewis came down from his training camp high in the Poconos mountains with the necessary adjustments made before producing a near punch-perfect display at the Mandalay Bay. He knocked Rahman out in the fourth to reassume his position atop the heavyweight division.

Nearly two decades on, parallels have been drawn between Lewis' Rahman incident and the fate which befell Joshua at the hands of Ruiz in New York six months back. Joshua, a huge favourite, was stunned by the rank outsider and his entire reputation has taken a hit as a result.

Just like Lewis did, Joshua has decided to dive straight into the rematch and one man who will be in the crowd at the purpose-built Diriyah Arena is Rahman, who has a heavyweight fighter of his own, Michael Hunter, on the undercard.

Now 47, 'The Rock' still cuts an imposing figure and he bristles a little at questions about Lewis. He does, however, appreciate the similarities between himself and Ruiz but told the new champion that now is when things start getting difficult.

“The mindset does change because you've now got somebody coming in trying to take it away from you now,” Rahman said of Ruiz. “It's quite frankly easier when you want to go and take something. Now you've got to defend something, you've got to stand your ground.

“But he is champion and I haven't seen any slacking off in training. He can fight and if he lands his punches you are going to see the same again.”

Lewis speaks openly about the handful of adjustments he made in his training camp which enabled him to deal with Rahman so effectively in the rematch. But the suggestion is that Joshua has made far more wholesale changes.

“I think Joshua is preparing to try to box and move and use his distance in the rematch,” Rahman added. “I believe Andy is preparing for a different Anthony Joshua in this fight. I didn't have that heads-up with Lennox. I think Andy is going to be OK in this fight.

“He is going to have to make adjustments. It's possible – he's been a world champion and he can box and that's what he has to do. But Anthony Joshua is so used to knocking everybody out that he has to remain disciplined and quite frankly make it a boring fight. I think he can win if he makes it a boring fight.

“If he tries to make it exciting and go for the knockout, then he gets knocked out. I think that is the key to victory for him. But for Andy, he's going to make sure it isn't a dull fight – let his punches go, use his hand-speed and combinations. I think Andy can repeat if he does that, but Anthony can be disciplined, use his height and reach and distance. It won't be pretty but he can secure a victory.

Rahman celebrates his victory over Lewis in April 2001 (AFP via Getty Images) (Getty)

“He's got that confidence that he wasn't hurt. You've just got to be able to adapt to the different style that Anthony can fight. He's not going to fight in the same way. It's been well documented that he's going to use his distance and boxing, and I think Andy is prepared for that.”

The one major question mark which appears to hang over Ruiz is how finally winning the world title has affected him. After years of struggling to pay his rent and bills, the 30-year-old is now able to splash with his new-found riches. With a reasonably quick turnaround between the two fights, it has been suggested that it is now Ruiz who might taken his eye off the ball.

“That can go both ways,” Rahman adds. “If you buy that stuff and you like continuing to buy that stuff, you've got to continue with your success. Floyd Mayweather liked buying all kinds of cars and bought them his whole career, and it wasn't to his detriment. He kept buying them and it gave him the hunger to keep winning so he could buy more.