The trouble left a 33-year-old man in hospital with injuries to his head and neck following the fight at The Circle Club in Manchester in the early hours of Sunday morning.
Diouf, a former player with Bolton, Liverpool and Rangers, was arrested at the scene on suspicion of violent disorder.
Five other men were also arrested and all have been bailed until May 23, pending further police inquiries.
Ferdinand, the brother of Manchester United and England star Rio, was questioned under caution by officers on Sunday.
Diouf and Ferdinand were team-mates when both played for Sunderland.
Police are investigating whether a bottle was used in the fighting.
A spokesman for Greater Manchester Police said: "A fight had taken place and a 33-year-old man was injured. He was taken to hospital for treatment to injuries to the side of his head and to the side of his neck, and he remains in a serious but stable condition.
"Six men were arrested on suspicion of violent disorder, five of whom have been bailed until May 23 pending further police inquiries. Another man has assisted the investigation, which is ongoing."
No-one at the Circle Club could be contacted for comment.