DANIEL DUBOIS tipped Tyson Fury to beat Anthony Joshua - having sparred both the heavyweight champions.
Fury, 32, and Joshua, 31, have verbally agreed a two-fight deal for 2021, as they eye an undisputed title decider.
The clash of styles and opposing personalties makes for Britain's biggest ever fight - with fans divided on the outcome.
But 23-year-old hot prospect Dubois, gives WBC champion Fury the slight edge against unified WBA, IBF and WBO king AJ.
Joshua knocked Kubrat Pulev out in the ninth round at Wembley on Saturday to set up the Battle of Britain clash.
And Dubois told : "Fury at the moment. His skill and his know-how, coupled with his confidence should beat Joshua.
"But it will be a really good fight. I wouldn't be surprised if it went the other way and Joshua won."
Dubois shared the ring with Fury when the Gypsy King was deep in depression and as a result overweight - but his talent had not left him.
He said: "He jumped in for one round with me in Bolton, just before I turned pro.
"I was sparring his cousin Hughie and we did a round together - he was really overweight back then, so it was just quick.
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"He's just as slippery as he looks though. He's a masterful boxer - he's learned his craft well."
Dubois earned a name for himself even before turning pro, following rumours he dropped a then-unbeaten Joshua in the gym.
But the soft-spoken prodigy remained tight lipped about the infamous spar, instead honouring the code of 'what happens in sparring stays in sparring'.
Dubois explained: " I want to build my own respect now.
"That was before I turned over and it gave me a great launching pad in the pro game - made everyone interested!
"All these promoters suddenly wanted to sign me... 'Who's this kid?' But now I want to build my own respect."