The Illini win came six days after Smith and freshman center Brian Carlwell suffered concussions in a single-car accident. The Illini struggled without Smith, who averaged 8.1 points per game this season despite being hampered by ankle injuries.
Tim Doyle, who had 16 points when the teams met Feb. 7, managed just 9 points and three assists for the Wildcats (12-14, 2-11 Big Ten).
Brian Randle opened scoring with a dunk off a pass from guard Chester Frazier, but Doyle answered with a layup, setting a basket-for-basket trend that would last through the half. Illinois led for 15:25 of the first half, but never pulled ahead by more than 7.
In the second half the Illini closed in on Doyle, double teaming him in the paint and holding him to 2 points. Northwestern scored just 10 points in the final 13:52.
Forward Warren Carter added 10 points and three rebounds for the Illini (19-9, 7-6), despite spraining his ankle in practice late in the week.