Members of the public and security rushed to the injured man’s aid and performed CPR on North Tce outside Roma Mitchell House, opposite the Adelaide Railway Station, about 11.50pm on Thursday.
Both men, who did not appear to know each other, had exchanged words before punches were thrown, witnesses said.
The man had fractures to his face and was taken to the Royal Adelaide Hospital where he remained in a stable condition. Police will speak to him once his condition improves.