One man who has been in the ring with both Anthony Joshua and Joseph Parker is English amateur Frazer Clarke.
Clarke, 25, has sparred over 60 rounds with WBA-IBF world heavyweight champion Joshua ahead of Saturday's title unification fight with WBO titleholder Parker.
Clarke, who flew to Australia last week to compete for England in the Commonwealth Games, trains alongside Joshua at his Sheffield training base and is hoping to box at his first Olympics in 2020.
"I've probably done over 60 or 70 rounds of sparring with AJ, I've been in the gym every week with him," Clarke told ESPN. "I've been sparring with AJ since the 2012 Olympics Games.
"So it's nothing new to me but it's great because as he's got better, I've got better. We've pushed each other on. It gives me a confidence boost ahead of any fight knowing that I've done rounds with the heavyweight champion of the world and a lot of tough rounds, no punches are pulled and it gets really heated.
"It's competitive, but heated at the same time. We're smart fighters trying to better each other. We did it for this camp, it's been a solid ten weeks of training. Started off with a lot of technical sparring, then we got to the open sparring and it got really interesting.
"I've seen him improve, I felt myself improve as well along the way. He's looking in good nick going into March 31. I've seen him improve and know how sharp he is. He's dangerous."
Clarke, from Burton-on-Trent, lost on points to Parker when the New Zealander was an amateur at the 2010 Commonwealth Games in India.
"I boxed Joseph 2010 in India," Clarke told ESPN. "It was a very good fight, close, and I lost out on points. I remember him being a very tough competitor, good engine and combination puncher.
"He's ferocious kind of guy who threw a lot of shots and came with a lot of intent. So these two styles are going to gel and it's going to be a great night."
Clarke, who is aiming to emulate Joshua's super-heavyweight gold medal for Great Britain at the 2020 Olympics, is tipping AJ to win by KO.
"For me, I've got to go with AJ," Clarke told ESPN. "Just from seeing him transform in the last ten weeks in training. I think the last two fights did him the world of good -- Carlos Takam and Wladimir Klitschko -- and that put him in a great place mentally.
"He learned from those and learned a lot of things hadn't done in his career up to then because the tests just weren't good enough for him. They were two real tests and he's come through them both so his experience, his youth, sharpness, fitness and strength all bodes in his favour.
"He's Olympic champion as well so he can box so I can see an AJ stoppage in the middle to late rounds."