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Mike Davis added 12 points and 10 rebounds for Illinois (8-2), while D.J. Richardson posted 12 points as well.
Illinois connected on 52.9 percent of its field goal attempts in the first half, including a 6-of-7 effort from three-point range, and led 48-25 at intermission.
Over the final 20 minutes, the Illini outscored the Broncos by 12 points despite missing all nine of their shots from behind the arc.
Strong defense was key to the win, as Illinois limited Western Michigan to 24.2 percent field goal efficiency.