The Badgers (5-1) never trailed and got a measure of payback against Duke (6-1) after being routed 82-58 two years ago in Durham, N.C. by ending the Blue Devils' 10-game winning streak in the annual event and the ACC's perfect record as well.
Kyle Singler scored 17 of his career-high 28 points in the first half and Andre Dawkins went 4 of 4 from 3-point range in the second half to finish with 12.
Mike Tisdale hit the go-ahead jumper with 41 seconds left, then blocked David Potter's driving shot on No. 18 Clemson's next possession as Illinois rallied from 23 points down for a the improbable victory.
The Illini (5-2) looked finished, down 51-28 early in the second half and with leading scorer Demetri McCamey saddled with three fouls and two points.
But Illinois finally got going with a 35-10 run to move in front in the school's biggest comeback ever.