In a quickfire round of questioning as part of a new Cosmopolitan magazine piece, Bieber offers up the Daughters singer's name when he's asked which artist he'd most like to work with.

That collaboration may have to wait until next year (16), after Mayer has finished his run of Dead & Company shows with a trio of Grateful Dead stalwarts.

The new quartet's first show isn't until Halloween night (31Oct15), but Mayer admits he's spending the summer perfecting the legendary rock group's complicated tunes.

He tells, "I've got what I call the double black diamonds. There are (songs) that require me to learn how to be a better guitar player. Slipknot, for example - if I tried to play it now, I couldn't do it, so it's forcing me to be better."

Meanwhile, the Baby singer also namechecks Mayer's on/off girlfriend Katy Perry, listing her song Teenage Dream as his favourite summer song in the Cosmopolitan interview, and admits the Beatles' Let It Be is his go-to karaoke tune.

Bieber also reveals he's terrified of spiders, hates girls in "gladiator sandals" and slow Internet, and he boasts, "​I can hold my breath underwater for two minutes."