Zac Brown Band are played Fenway Park Friday (June 27) and Saturday (June 28), but that didn't stop the group's frontman from showing up early, surprising concertgoers at the Billy Joel show on Thursday.

The 'Piano Man' hitmaker was treating the crowd to an encore when he called out Brown, much to the delight of fans. "I'm playing one night at Fenway Park, but this guy is playing two," Joel said as Brown walked onstage with a huge grin, clearly pumped to play with the music legend.

Brown and Joel sang 'You May Be Right,' trading off verses and harmonizing throughout the rowdy collaboration.

The country group was one of two surprise guests that evening; Emma Stanganelli, a 13-year-old musician, accompanied Joel on 'Boston State of Mind.' The 24-song set also included 'Sweet Caroline,' the Lovin’ Spoonful’s 'Summer in the City,' and 'Take Me Out to the Ball Game,' which was perfect for the setting.

Zac Brown Band have been highly anticipated at the infamous baseball stadium, where they sold out not one, but two nights at record speed. The country group hadn't even announced the opening acts before both shows were completely sold out.

In addition to Fenway Park, ZBB are on a lengthy summer tour, the Great American Road Trip Tour, running through Aug. 15. They'll also hold their fourth annual Southern Ground Music & Food Festival in Charleston, S.C. this fall, closing out each of the two nights with concerts, with some of their bandmembers also showcasing their solo work.

Who knows, maybe Joel will show up at one of their shows and join them for a rendition of 'Piano Man,' a favorite song ZBB often cover!