Joshua is set to put his IBF, WBO and WBA on the line against the undefeated Usyk at the Tottenham Hotspur Stadium on Saturday 25 September.
The heavyweight world champion took the fight after Tyson Fury was ordered to face Deontay Wilder for a third time, delaying the chances of their unification bout.
A meeting with Fury would move closer if Joshua defeats Usyk in his first fight since December, but AJ insists the 34-year-old Ukrainian will be the second-toughest opponent of his career so far.
While Joshua won heavyweight gold at the London Olympics in 2012, Usyk won the cruiserweight title before going on to dominate the division in the professional ranks. He has since made the step up to heavyweight and is aiming to become a two-weight world champion.
“On paper he sits number two, behind Wladimir Klitschko,” Joshua told BBC Radio 5 Live.
“They’re very different technically, but on paper he’s an Olympic gold medallist, he reigned for over 10 years, he’s number one.
“But Usyk is number two because of his success at cruiserweight and then his success so far at heavyweight.”
Joshua knocked out Klitschko at Wembley Stadium in 2017 and defeated the likes of Joseph Parker and Alexander Povetkin before losing his belts in a shock defeat to Andy Ruiz Jr in 2019.
He regained his titles six months later before beating Kubrat Pulev last December, with Usyk sat at ringside.
“One of his strengths is that he’s getting in the ring with me. I’ve got to give him credit for that, it’s going to be tough,” Joshua said of his upcoming opponent.
“He’s a southpaw, he’s been fighting orthodoxes his whole life, we fight southpaws once in a blue moon.
“I’m doing a lot of ring work, certain fighters do a lot of road works, pads, bags, shadow boxing.
“But with a southpaw it’s all to do with positioning and we’re doing ring work, a lot of southpaws are counterpunchers.”
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