The 45-year-old musician opened up on his relationship with the former big man during Walton's "Throw It Down" alternative NBA broadcast on Monday ... explaining how the die hard Grateful Dead fan gets him ready to hit the stage before shows.

"When Bill gives you advice, he looks you dead in the eye and everything stops," Mayer said.

"He speaks with this cadence -- 'Go out there. Give it your absolute all. Don’t be shy. They are here to watch you play. Go play to the greatest of your ability ... and some of the greatest compliments I’ve ever been given are from Bill Walton."

Of course, Mayer plays guitar and sings for the GD spinoff band -- so Walton has seen him perform countless times. He even took the stage with the band in 2015.

In fact, Mayer says Walton -- who stands at 6'11" -- is easily recognizable in the crowd ... referring to the guy as "air traffic control" for the fans in attendance.

Bill explains his reasoning for holding his hands up in the sky during shows ... saying he spent 4.5 years immobile with a deteriorating spine, and now that he's able to move -- he's making the best of it.