Jelly Roll thought it was "a joke" when Eminem's team reached out to ask him to perform with the rap legend.
The 39-year-old country star and rapper had to pinch himself when he was invited by the 'Lose Yourself' hitmaker, 51, to perform 'Sing for the Moment' with him as a surprise for this month's 'Live From Detroit: The Concert at Michigan Central' special for NBC.
Speaking to 'Entertainment Tonight', he said: "When I think about coolest moments of my career, right now at the top, there has to be this thing that I got to go sing with Eminem in Detroit.
"I got to sing 'Sing for the Moment' with him, which is a record where he sampled Steven Tyler. I mean, just what an incredible night and I got to go do it in Detroit. It was unreal."
The 'Son of a Sinner' star - whose real name is Jason Bradley DeFord - dubbed the duet the "coolest moment ever".
He recalled: "Em reached out, his team reached out and said, 'Would you be interested in doing this since he was already there doing the secret tribute?' I couldn't believe it. I thought it was a joke until I met Eminem himself.
"I thought I was being joked! I even know Eminem's manager. He's a friend of mine. He's really good to me. His name is Paul and I was like, 'Paul, don't play.' He was like, 'I swear.' And as soon as I met Eminem, it was like the coolest moment ever, man."
He confessed: "I was giddy, like a child.
"You could see it all over my performance. Just the kid in me. I thought the camera was off of me. So as soon as I get through singing, I'm like, whoa, I just let this steamroller out. It's really cool."
