Anthony Joshua has 'never faced' anyone like Oleksandr Usyk before, according to the Ukrainian's promoter Alexander Krassyuk.
The former undisputed cruiserweight champion will challenge the unified heavyweight king at the Tottenham Hotspur Stadium in London on September 25.
And Krassyuk, who has promoted Usyk since he turned professional in 2013, has warned Joshua he will face the 'biggest challenge' of his career to date.
Speaking to MyBettingSites, he said: "The fight with Ruiz can be some sort of guide for any fighter, but AJ didn’t look proper in that fight.
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"It was his debut in the United States, it was a late replacement of the opponent and he wasn’t prepared for that kind of fight.
"He lost by chance and then in just three or four months he corrected his mistake easily and got his belt back.
"Joshua only lost once, but Joshua faced a southpaw only once when he first became world champion.
"Let’s be fair, Charles Martin was lucky to get his belt without victory because his opponent was injured and didn’t make it to the end of the fight.
"Eddie Hearn was very smart to get that fight for AJ and make him world champion.
"That was his only experience as a professional facing a southpaw, but he’s never faced a skilled, high boxing IQ southpaw, and I think it’s going to be the biggest challenge for AJ."
'He is working really hard in his training camp'
Some boxing pundits have cast doubts over Usyk's chances of upsetting Joshua, pointing to the obvious differences in terms of size and weight, with David Haye among those predicting it will be a 'devastating' night for the Ukrainian.
However Krassyuk, who co-founded K2 Promotions with Vitali and Wladimir Klitschko, insists that everything is going according to plan.
"The preparation is going brilliantly," he added.
"Usyk spent April and May working really hard.
"Now when you look at him, you don’t hesitate anymore and wonder whether it’s heavyweight or cruiserweight in front of you: it’s 100% heavyweight.
"He did a really great job with his functional muscles; now he is working really hard in his training camp and just coming to the end.
"He’s had many sparring partners, a lot of experience inside the ropes. He is so tired of the work but he is so inspired that you can just recharge the batteries."