Former undisputed world heavyweight champion Mike Tyson has claimed Andy Ruiz Jr is making a mistake by dropping the pounds ahead of his much anticipated rematch with Anthony Joshua.

Ruiz defeated Joshua on June 1 by technical knockout in the seventh round, capturing the unified WBA/IBF/WBO/IBO titles.

With the rematch set for Saudi Arabia in December, the Mexican-America's decision to slim down has been criticised by legend Tyson.

“There have been rumours he has lost weight.

“I don’t like that.

I believe losing the weight can affect power sometimes.

“If it’s not broken, don’t fix it.

“He did well at that weight, so he should stay at that weight."

Having replaced Jarrell Miller for Joshua's debut fight across the Pacific, Ruiz Jr now holds 33 career wins with just one blemish on his record, a defeat to Joseph Parker in 2016.

Tyson added: “In the first fight, Ruiz put the pressure on him so fast that he didn’t know what to anticipate.

“And there were some hard punches.”

Speaking on Tyson's podcast, Ruiz Jr was already looking beyond his first defence against Joshua: "I know I became the champion of the world and I want to defend it a lot of times.

“I want to fight Tyson Fury.

“But in life after boxing – my dad’s a contractor, so I want to build an empire in construction and build a lot of houses over there in my hometown.”

Ruiz shocked the world with the biggest upset victory since Tyson was knocked out by Buster Douglas in Tokyo back in 1990.

In a similar underdog story, Ruiz Jr fought the odds against a more physically imposing champion and will look to do the same and prove his first victory over AJ was no fluke.

The 'Destroyer' is set to defend his world heavyweight titles in a rematch against Joshua in December.