Fans at Dierks Bentley's first of two shows at Red Rocks Amphitheater in Colorado on Monday night (Sept. 26) were treated to one heck of a surprise during the show: When it was time for Bentley to sing "I'll Be the Moon," he welcomed Maren Morris to the stage.

Morris sings with Bentley on the studio version of "I'll Be the Moon," which appears on his most recent album, Black, but her appearance at Red Rocks was the pair's first time performing the song together live in concert. Morris has been on the road with Keith Urban, as an opening act on his Ripcord World Tour, and, according to Bentley, flew into Colorado from Boston to make her appearance.

Readers can press play above to watch Bentley and Morris' duet. After singing "I'll Be the Moon" together, Morris stayed onstage to perform her own song "My Church" with Bentley, as well as Bentley's tune "Bad Angel," for which Randy Houser joined the pair as well.

With Morris on "I'll Be the Moon," the track is a perfect duet -- but as Bentley explains, it wasn't always like that.

“We first recorded it just me, but the song looks at a relationship from a different angle, where the girl is instigating the deceit. So, I felt like it needed a female voice, and I started to hear it more as a duet,” he tells Rolling Stone. “I love the perspective Maren adds; there’s weight to her voice.”

Bentley's 2016 Somewhere on a Beach Tour is scheduled to continue through the end of October.

See Pictures of Dierks Bentley + More Country Stars Then and Now

How Well Do You REALLY Know Dierks Bentley?