The Blue Demons took a 2-0 lead in the top of the third on former Oak Park outfielder's Alex Morocco's two-run single.
But the Illini came back with two out in the bottom of the fourth when former Sandburg star Ashley Conrad hit a triple down the right-field line to drive in two runs and scored on a throwing error.
Illinois added an insurance run in the fifth on Hollie Pinchback's sacrifice fly .
The Blue Demons threatened and had runners on second and third with two out in the top of the seventh when Tara Voss fouled out.
Illinois will face the Missouri-Creighton winner on Saturday and DePaul will face the loser as the double-elimination regional continues.