Day allowed one earned run, three hits and did not issue a walk as ISU improved to 21-5.
The Redbirds scored three second-inning runs on a two-run double by Mack Leonard and a Kaili Chval single that drove home Leonard.
ISU added four runs in the sixth.
Spenser Strandgard and Brandi LaFountaine blasted solo home runs before an RBI double from Reili Gardner and a run-scoring Leonard single.
