The former Nirvana bassist, whose song 'In Bloom' features in the Harmonix title, said a child beat him playing the virtual version of the track.
"I had never tried the game before, so I gave it a go. I worked through the menu and found the song 'In Bloom'. I picked up the little guitar-shaped controller and hit the stage," he wrote in the Seattle Weekly.
"I couldn't quite master this new, different way of playing it. I tried to hit the notes cascading down the screen, but could barely keep up.
"Meanwhile, this kid was watching me fumble with the game. I became self-conscious and took the controller off. I handed it to him, and he proceeded to jam on the song, and was really good!"
He added: "He had no idea that I was the musician he was emulating on the game, and I didn't tell him."