Floyd Mayweather is "leaning towards" Errol Spence Jr taking Kell Brook's world title away.
The former five-weight world champion is the latest American to back the IBF welterweight challenger ahead of Saturday's showdown.
Mayweather sparred southpaw Spence Jr in 2013, and has seen enough to suggest he could upset Brook in Sheffield.
However, at the Kong Events photo session with world heavyweight champion Anthony Joshua at the weekend, 'Money' admitted taking Brook down the stretch will be his best chance.
"Errol is a hell of a fighter, we sparred and he gave me good work," Mayweather told Sky Sports.
"If he can fight in the later rounds I lean towards him. I know your guy Brook can fight and boxed real good against Triple G, so we've got a great fight on our hands."
Mayweather, who turned to Spence Jr ahead of the defence of his WBC welterweight against Robert Guerrero in 2013.
Spence Jr was two fights into his professional career but has had 'Money' supporting him, even if he is not at Bramall Lane on Saturday night.
"Ever since then (the spar) he [Mayweather] will call me on Facetime, he will text me to check up on me and see how I'm doing and stuff like that," said Spence Jr. "We're good."