Nick Carter's looking for aspiring musicians to serve as his opening act on a nationwide tour ... and the first honor goes to a guy who served him coffee.

The Backstreet Boys singer just started the U.S. leg of his "Who I Am" world tour and he needed an opener for Tuesday's show in Cleveland, OH ... and he found his man at a local restaurant.

Nick was grabbing coffee before the show when he started talking to his server, Jordan Tellez ... and when the dude showed him his music on Spotify, Nick asked him to join him at his show.

TMZ obtained video of Nick introducing Jordan to the crowd at his Cleveland concert ... and Tellez took the opportunity and ran with it ... as you see in the clip.

Nick's running a social media competition that's his little version of "American Idol" ... and Jordan's setting the bar here.

For his part, Jordan says the whole experience with Nick was "unreal" ... and it will be interesting to see who else Nick discovers in North America.