Jason Sudeikis -- Only the Good Sing Billy Joel Covers

It was nine o'clock on a Saturday ... an unsuspecting Kansas City bar crowd gathered round to watch a popular cover band ... but they got a big bonus ... Jason Sudeikis.

The SNL star and his fiancee Olivia Wilde hit up Kanza Hall ... and Jason joined the band Summer Breeze for some classic hits -- including Billy Joel's "Just the Way You Are."