The Badgers (15-7, 6-3 Big Ten) shot under 30 percent for most of the first half and combined with the Illini to score 14 points in the game's first eight minutes.
Wisconsin eventually started making shots and Illinois (15-8, 2-7) didn't until it was too late. The Illini closed the gap late but trailed by 15 with 5:13 to play.
Frank Kaminsky added 19 for the Badgers and Ryan Evans scored 15. The win keeps the Badgers in fifth place in the Big Ten.
Illinois was led by Joseph Bertrand's 17 points. Four other Illini scored in double figures.
The Illini have lost seven of nine and have fallen to 10th in the conference.