The top-seeded Hoosiers (27-5) are off to a good start after claiming their first outright conference title in two decades and earning a first-round bye.
They took control midway through the first half and withstood a mild push in the second, avenging a two-point loss at Illinois (22-12) last month. That set up a matchup with Michigan or Wisconsin in Saturday's semifinals.
Zeller had another big game after scoring a season-high 25 in a win at Michigan. He was 9 of 11 from the field and hit all six free throws.
Victor Oladipo added 12 points and 11 rebounds and drew the loudest cheers with a 360-degree fastbreak dunk with 90 seconds left. The Big Ten's Defensive Player of the Year, he also helped contain Brandon Paul.
Christian Watford scored 15, and the Hoosiers shot 54 percent overall even though they were just 5 of 17 on 3-pointers.
They were able to get good shots, though, because they locked up the Illini on defense and dominated them on the glass 38-26.
Paul finished with 16 points, but most of them came from the free throw line, where he was 11 of 12. Otherwise, it was a rough afternoon. He shot just 2 of 13 after scoring 25 and hitting the winning jumper at the buzzer in the first round against Minnesota.
Tracy Abrams scored 16, but Illinois was out of sync at the start and couldn't make up for it down the stretch.
The Illini got within single digits for the first time since the early going when D.J. Richardson hit two free throws with 8:17 left to make it 60-52, but Indiana held its ground.
Jordan Hulls whipped a pass to Oladipo underneath for a layup after Abrams missed a 3, and the Hoosiers struck again after Richardson missed from long range. This time, Watford hit from beyond the arc to make it 65-52 with 5:31 remaining, and the Hoosiers fans let out a loud roar and maybe a sigh of relief.
The Hoosiers clicked into gear after a sluggish start and were up by as much as 17 while building a 35-21 halftime lead.
Zeller dominated in the early going, scoring 14 in the half and keying a 12-2 run that bumped their lead from five to 22-7. He scored eight during that stretch, punctuating the run with a put-back and a fastbreak finger roll with 7:33 left after he stole the ball from Paul.
He also finished the half with a neat basket, grabbing the ball in midair and redirecting it into the net after Jeremy Hollowell had his shot blocked by the Illini's Nnanna Egwu.