Tony Bellew has predicted Anthony Joshua will make his superior size count in the later rounds against Oleksandr Usyk later this month.
Joshua will put his WBA, WBO and IBF world titles on the line against former cruiserweight Usyk at the Tottenham Hotspur Stadium on September 25.
AJ posted a picture of himself in training this week on his Instagram account with the caption "Heavy hitta".
The 31-year-old looks in superb shape and, with the mentioned caption, he is obviously hoping to make his size count against his smaller opponent.
His close friend Bellew, a man who was on the wrong end of a battle with Usyk in November 2018, knows exactly what Joshua is up against.
Bellew retired after the eighth-round stoppage to Usyk in Manchester and has said he is the best fighter he faced in his career.
The former WBC cruiserweight champion has reiterated that Joshua faces a fighter who is more than capable of causing an upset.
Rounds seven, eight and nine key
"This will be the best boxer Anthony Joshua will ever face in his career," Bellew told talkSPORT.
"When it comes to natural, pure boxing, Oleksandr Usyk is the most natural boxer Anthony Joshua will ever face. He is unbelievable at what he does.
"He is not a true heavyweight. Everyone keeps going on, yes he is a cruiserweight, but he is the greatest cruiserweight who has ever put on gloves. So he does come with an amazing skillset, but I just think weight divisions exist in boxing for a reason."
Bellew thinks that Joshua's endurance and experience at heavyweight will lead him to victory.
"Come rounds seven, eight and nine, he is going to find out why he is not a true heavyweight. I think Anthony Joshua's speed, power and perseverance will be what catches up with Oleksandr Usyk.
"I wish he would stop fighting my friends because I want to support him! I'm a huge admirer of him and I have so much respect for the boy, but he just keeps fighting my friends so I can't support him."
AJ Sparring with McKean
Joshua (24-1) has brought in Australian heavyweight Demsey McKean for some sparring ahead of his fight with the unbeaten Usyk.
McKean, 30, who like Usyk is a southpaw, says he has been drafted in to prepare AJ for the razor-sharp skills of the Ukrainian.
"I'm up in Sheffield for the next two weeks, joining AJ's training camp," he told Sky Sports earlier this week.
"Done some training yesterday afternoon. He was there, met him, had a bit of a chat. A lovely bloke. We've got our first spar today. I'm looking forward to that. It's going to be a good experience.
"He's the biggest name in boxing in the world at the moment, in all weight classes, let alone heavyweight. To be in there and to gauge myself against the No 1 guy in the heavyweight division, that's massive for me.
"A really good experience. It was good to be training alongside him and it's going to be even better to trade some punches with him very shortly."