Last updated on .From the section Boxing

Ruiz (left) recorded a fifth-round technical knockout against Alexander Dimitrenko

Britain's Anthony Joshua, 29, says his next opponent Andy Ruiz Jr is at "championship" level and he has "not underestimated him one bit".

Unbeaten Joshua will make his American debut in a unified heavyweight world title contest at Madison Square Garden in New York on 1 June.

The suitability of his opponent has been called into question, but Joshua says Ruiz "is a great challenger".

"It's not about what you look like - it's a craft, a skill," he said.

"What's in your heart and head matters in the end. Andy has shown he has all that, he can fight and box, that's what matters.

"I think Andy is a great challenger and will bring it on 1 June," added the IBF, WBA and WBO champion.

Joshua has defended Ruiz (pictured) following criticism of his physique

California-born Ruiz Jr, 29, has lost only one of his 33 bouts, which came on points in his title fight with Joseph Parker in Auckland in 2016.

He beat Germany's Alexander Dimitrenko on 20 April after weighing in at 262lbs, 17lbs heavier than Joshua was in his last win over Alexander Povetkin in September.

But at 6ft 2in, he is four inches shorter than Joshua and will be at a substantial reach disadvantage.

"All I've heard is 'AJ will smash him in a round'," said Joshua, who has claimed 21 of his 22 professional wins by knockout.

"From the outside they think that anyone can box, so how is the guy that doesn't look like a fighter able to box?

"I always say that if you put 10 bodybuilders in the ring, not one of them could fight for a regional title in boxing.

"Take me out of my body but keep the same attributes and height, same jab, same chin, same heart and same mind, but I looked different - I'd still get to the same position I am in because it's what is within you that makes a champion.

"He's keen, he's game and you cannot knock him. He can fight and he's got hands. He gave a world champion in Joseph Parker lots of problems. Andy is championship level for sure, and I have not underestimated him one bit."