The Yeah! hitmaker was so determined to get down to the boxer's fighting weight for the film he bought himself a bike and opted to cycle the 10 miles (16 kilometres) to his Atlanta, Georgia gym for his strenuous workouts with revered trainer Xavier Biggs.
But he quickly realised his body wasn't quite ready for such a hardcore regime.
Usher tells Men's Health magazine, "My second day I decided to ride to the gym. So I rode there. Boxed. Rode the bike back home.
"Mentally, I'm the type of person who can ignore the pain. Put it somewhere else and allow my body to just go there, but I always suffer.
"You can't box, ride a bike for 20 miles in the heat - Georgie (sic) heat - and then go to the studio and rehearse. That's not happening."
Exhaustion aside, Usher is determined to become just like the boxing icon he is portraying on film after spending the best part of a year getting into shape.
He adds, "My commitment to this character demands that I not only physically get ready but also become him. The hair, the personality, the manner he conducts himself in - all that. He's an incredible man."
Roberto "Manos de Piedra" Duran was fierce in the ring, his poor upbringing on the...
When Justin Bieber was discovered by Scooter Braun and Usher Raymond after performing his songs...
Clearly designed with his 12-year-old female fanbase in mind, this backstage myth-making doc wants to...