Zac Brown's friendship with the Foo Fighters' Dave Grohl continues to blossom, as the country singer joined the rock group for a classic performance of Black Sabbath's 'War Pigs' on Monday night (Oct. 13).

The Foo Fighters are doing a weeklong stint on 'The Late Show with David Letterman' and for their first performance of the week, they called on the Zac Brown Band's frontman for help

"I'm told that this particular song is not on the CD but it's just something a little special," Letterman explains in the clip, "But we're not telling Zac that it's not on ... Whoops! I've gone and done it again. Dang it."

As soon as the drum kicks in, Brown looks every bit a rocker (especially in his fancy hat), playing guitar while leading vocals on the Black Sabbath hit. The country singer seems incredibly comfortable performing a rock classic next to some of the best rockers in the business, while Grohl stands next to him, tossing his hair in true rock n' roll form.

Love the collaboration or hate it, it's definitely fun -- and Brown's vocals fit the song perfectly. The studio audience seemed to agree, as the performance was met with loud cheers. Letterman was also impressed, walking to the band to shake hands while singing their praises.

The host summed it up well when he said: "Oh my God ... Zac Brown. Black Sabbath. Foo Fighters. I don't know, what do you kids want from me?"

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