The loss drops the Badgers into a second place tie in the Big Ten with the Illini. The teams have identical conference (8-3) and overall (17-5) records. Michigan State leads the league at 9-3 in conference play.
Williams was the offensive sparkplug Wednesday, scoring 12 first-half points when the Illini were struggling to find the net and hitting a huge shot late in the second half to effectively end the Badgers' comeback hopes.
Wisconsin never led in the second half, but drew within 50-49 with five-and-a-half minutes to play after Zach Morley's 3-point shot capped a 7-0 run.
The Illini, though, answered, ripping off an 8-0 run in a two-and-half-minute span in which Wisconsin took just one shot and turned it over twice. After two James Augustine free throws made it 52-49, Williams hit a long 2-point shot. Roger Powell added a putback and Williams hit two free throws to cap the run.
Wisconsin never drew closer than seven the rest of the way, as Illinois made 7 of 11 free throws, with Williams connecting on 6 of 6, in the final three minutes.
The Badgers led throughout much of the first half and built their biggest lead when Devin Harris hit a 3-pointer to make it 18-12 with 8:12 left in the opening stanza. Williams, though, hit two 3-pointers to draw Illinois within 20-18. The Badgers pulled back ahead by six, but Dee Brown knocked down a triple from about six feet beyond the arc to spark a 12-4 run that gave the Illini a 30-28 lead at halftime.
Prior to Brown's jumper, Illinois had made just 5 of 17 field goals. The Illini hit all five of their shots during the 12-4 spurt before missing two opportunities in the last five seconds.
Williams finished 5 of 9 from beyond the arc and 8 of 8 from the free throw line, pacing an Illini effort that included 8-for-19 3-point shooting and 21-for-27 free throw shooting, the most free throws made and attempted by a Badger opponent all season.
Brown scored just four points but led Illinois with six rebounds and six assists.
Harris paced Wisconsin with 16 points and seven assists but also committed a game-high six turnovers as the Badgers gave the ball up a season-high 17 times. Zach Morley was also in double figures with 12.
Illinois constrains Harris, Badgers - sidebar