The Flyers opened a 23-9 advantage and held off the Fighting Illini, who became the fourth and final No. 1 seed to be eliminated from the tournament. Arizona State, Mississippi State and Virginia Tech were also given No. 1 seeds in the event.
Dayton will meet Mississippi next Tuesday in the semifinals, which will be played in New York. North Carolina and Rhode Island will be in the other semifinal.
The Flyers became the third team out of four to win a quarterfinal contest on the road.
Johnson scored 18 points and the Flyers (23-12) made 8-of-18 shots from 3-point range. Illinois (21-15) was led by Demetri McCamey with 13 points.
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