Fighting Illini guard Rebecca Harris led all scorers with 22 points and got support from backcourt mate Lori Bjork (14 points) and forward Jenna Smith (13).
The Buckeyes countered with 19 points from guard Marscilla Packer, but she was the only perimeter threat Ohio State could mount.
With the offense bogging down, Packer and fellow guard Ashlee Trebilcock combined to take 29 shots, some of them earlier in the shot clock than OSU head coach Jim Foster would have preferred. Packer made 6 of 16 while Trebilcock was 3 for 13 and finished with seven points while gutting out 28 minutes despite a back injury.
Despite the seeds of the respective teams, the outcome could not have come as a terrible shock given how the regular season series played out.
In Columbus Jan. 17, Illinois controlled much of the action until Ohio State escaped with a 43-42 win thanks to a late three-pointer by Maria Moeller.
Two weeks later, the Fighting Illini gained a measure of revenge with a 68-64 win back in Champaign.
With their latest win over Ohio State, they became the first No. 9 seed to knock out a No. 1 in the Big Ten tournament. Ohio State failed to win at least one game in the tournament for the first time since Foster took over the program in 2002.
Ohio State wanted to end the season with the last laugh over the Illini - and perhaps later Michigan State - but will not get the chance.
"We definitely expected them to play hard and play like they did," Packer said. "Obviously we expected to win the game, but they played great. Harris did well for them and so did Bjork."
Bjork made 4 of six three-point attempts, while Harris did her damage both inside and out. She was 2 for 5 from three-point range also got into the lane a few times as Ohio State point guard and Big Ten defensive player of the year Shavelle Little struggled with an injured left knee.
Little did not score in 15 minutes of action.
"I thought their quickness at the guard spot hurt us a little bit, but our guards playing fast hurt us more," Foster said.
"I think we got into a period where I thought we were taking very quick shots, ones that were not to the level of quality that you would like to take. I think a floater is a nice shot to have in your game, but I don't think it's a nice shot to take with 22 seconds to go on the shot clock.
"I think we rushed at the guard position and weren't patient enough to take advantage of our inside presence. Tam and Jantel's lines are terrific and more shots would have helped."
The rubber match got away from Ohio State at the start of the second half.
The Buckeyes led by a point at the intermission – thanks to a last-second three-pointer by Alison Jackson at the first-half buzzer – but stumbled out of the gate in the final 20 minutes.
They committed four turnovers before scoring a point while Illinois scored six in a row to go ahead 33-28.
Trebilcock momentarily put a finger in the dam with a three-pointer from the left wing with 16:58 to play, but Ohio State remained in trouble. The offense never quite got in sync, and by the 13:41 mark, the deficit was eight.
With eight minutes to play, the Buckeyes were down 10, but they did manage one final push.
After cutting the deficit to three, the Buckeyes had two chances to tie the game in the final 35 seconds but could not do so.
Packer gave them their first opportunity with a steal, but her three-point attempt with 32 seconds to go was off the mark.
Lavender rebounded that miss but could not make a contested shot. Riley gathered in Lavender's miss, and the Buckeyes called timeout to talk things over with 21 seconds on the clock.
Coming out of the timeout, Ohio State never got a good look at the basket but clung to slim hopes nonetheless. Trebilcock missed an off-balance jumper, but Riley went to the foul line after a loose-ball foul on Lacey Simpson.
The OSU senior made the first freebie but missed the second, and Bjork made two free throws to essentially sew up the decision.
Harris hit a pair of free throws of her own to set the final margin.
Bjork, Harris and Chelsea Gordon all played the full 40 minutes for Illinois (18-13), while Smith logged 39.
Packer went the full 40 for Ohio State.