Sir Paul McCartney "lost it" when he got to perform with Brian Wilson.
The Beatles legend sang at a charity concert on the Beach Boys star's hit 'God Only Knows' and admits he was very overwhelmed by the opportunity.
He explained: "We were doing a benefit together, and I was OK on the actual performance, I held it together, but at the sound check I lost it, because it's very emotional, this song, I find it.
"I can sit and think 'Oh my God, I'm singing it with Brian', it just got me. I couldn't do it."
Paul admits he and bandmate John Lennon used to compete with songwriting, not just with one another but also the Beach Boys, and thinks it inspired them to do better.
Speaking on Absolute Radio's Ronnie Wood Show, he added: "Brian Wilson sort of proved himself to be like a really amazing composer, and I was into chords and harmonies and stuff at that time, and we ended up, it's kind of like a rivalry; we'd put a song out, Brian would hear it, then he'd do one, which is nice.
"It's like me and John, you kind of try and top each other all the time. But he eventually came out with 'God Only Knows'."