The Cruise hitmakers were one of the many superstars on the line-up for the Las Vegas event, and during their set at the T-Mobile Arena, they broke out into the chorus of the boy band's hit, Everybody (Backstreet's Back), a track the duo had covered multiple times on tour.
But this time, they unexpectedly introduced the quintet to the stage to join them, much to the delight of the thousands of screaming fans in the crowd.
After Brian Kelley and Tyler Hubbard finished their performance, the Backstreet Boys returned to the stage for a mini-set, singing I Want It That Way, As Long As You Love Me, and Larger Than Life, which is also the name of their newly-announced residency at The AXIS Theater at Planet Hollywood Resort & Casino, which will begin in March (17).
While the surprise performance was the first time the singers had taken the stage in front of a live audience, their collaboration dates back to earlier this year (16), when Kelley and Hubbard's BSB concert covers caught the attention of Nick Carter, and they promised to hit the studio for a duet.
The country duo recruited the group to sing a track titled God, Your Mama, and Me, which features on Florida Georgia Line's new Dig Your Roots album.
Both Kelley and Hubbard have never been ashamed to public discuss their love for the boy band, and speaking of the project to The Huffington Post, Kelley noted they were in awe after working with their idols, calling their duet a "bucket list collaboration".