A bouncer who was questioned after a man was punched to the ground during an argument outside a Brisbane pub has been released without charge.

The 21-year-old patron remains in a critical but stable condition after he was knocked unconscious about midnight.

Police say the man was punched while trying to re-enter the bar in Bulimba during a clash between two groups.

Bouncers separated the feuding parties following the blow and the man was taken to the Princess Alexandra Hospital.

A 26-year-old security guard was interviewed following the incident and released without charge, police said on Sunday.

AAP