TMZ broke the story ... the Madison Police Department is investigating the July 27 incident ... and believe it may be related to the campus attack on Ball which occurred two days later.
Cops initially LIED to the media, insisting they were NOT checking into the fight ... and even blasted a TMZ story in which we said detectives were following up on tips about the melee.
Of course, the MPD later admitted they were completely full of crap ... and now believe they have evidence proving Ball WAS at the house party during the July 27 beating.
TMZ has learned ... the alleged victim met with investigators on Friday ... and told them Ball, along with other members of the University of Wisconsin football team, beat him so badly, he suffered a concussion, bruises and his tongue was split open.
The alleged victim took a cab to the hospital the night of the attack ... but never went to police.
Now, TMZ has learned ... two of the alleged victim's friends were interviewed by police this weekend to see if he ordered a retaliatory strike on Ball.
Ball insists he was not involved in a fight on July 27.
So far, no comment from UW.