Eminem, real name Marshall Mathers, has been working with trainer Emanuel Steward at a gym installed in his deluxe Detroit mansion.
Steward said: "I put him through his paces and I was amazed that he was as good as he was. Since then, he's trained regularly and I've put him in the ring with some top-notch sparring partners. He's improved enormously. He's aggressive - a real slugger... He moves a lot like Muhammad Ali."
It is believed Mathers is honing his skills for a forthcoming movie role, following his award-winning performance in the hit 2002 film "8 Mile".
He has been linked to a big screen biopic of the undefeated 22-year-old Ukranian-born Jewish boxing sensation Dmitry Salita.
However, studio bosses have been recently reported as saying that Mathers' fondness for tattoos could hinder future acting parts.
A Universal studio insider said: "People are really starting to worry, because he keeps talking about what his next set of bodywork will entail. He is hard enough to cover up properly now, but if there are any more it will just be ridiculous and it will seriously affect him winning parts."
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