Oleksandr Usyk can put 'doubts' in the mind of opponents with his 'sensational' skills, says Isaac Chamberlain, who sparred numerous rounds with the Ukrainian star.
Usyk is awaiting news on his next fight after securing the WBA, IBF and WBO heavyweight titles with a masterful points victory overt Anthony Joshua in September.
Chamberlain was twice called into training camps to spar with Usyk after impressing the supremely talented southpaw and was fully aware of the threat posed to Joshua.
"You know what, I wasn't really surprised. I know how good he is," Chamberlain told Sky Sports.
"He's a sensational boxer. I've been in two camps with him.
"It's the subtle things that he does.
"The way he sets up attacks, feinting all the time, lead hand all the time. He's using his boxing brain a lot. He's very, very good at what he does.
"I did very, very well against him. They sang my praises, they said that I would definitely be coming back.
"I used to ask Usyk for advice all the time after the spars, but obviously when I would spar a bit better, he would stop giving me advice."
The British contender helped with preparations for world cruiserweight title fights against Mairis Briedis and Marco Huck before Usyk stepped up a division in pursuit of even more belts.
Usyk is now set to press ahead with an immediate rematch against Joshua after rival champion Tyson Fury was ordered to make a mandatory WBC title defence against Dillian Whyte.
"With me, he would attempt to punish me. Luckily, I was very, very quick," said Chamberlain.
"He's very smart with his approach. He does everything well. He's a proper student of the game.
"If you keep getting countered, of course you're going to have doubts like, 'If I throw something again, I'm going to get countered again.'
"Boxing didn't work last time, he [Joshua] has to stick it on him."
Chamberlain returns to action himself this Friday as he fights Dilan Prasovic for the vacant IBF international cruiserweight belt at Crystal Palace.
The south Londoner hopes that victory will propel him up the rankings towards a showdown with IBF champion Briedis.
"That's the plan," said Chamberlain. "He has the IBF and the Ring Magazine [belts]. I want to go straight for No 1.
"He's getting old, but he's the best out of everyone, so I want to challenge myself against the best in the world. That's what we're going towards.
"He's a come forward fighter, he's very smart, calculated aggression and I believe if I keep going and keep working, and we apply the right tactics, then I can definitely beat him."