Raymond Kasper, 49, is on trial in McHenry County on charges of predatory sexual assault and aggravated criminal sexual abuse against a girl under the age of 13.
The girl testified in the first full day of testimony Tuesday she dreamt the whole thing, the Chicago Tribune reported.
Prosecutors said the assaults took place in 2011 between June and October.
Prosecutors said they believe the girl may have been coerced into changing her testimony and played a recording in court of a telephone call between Kasper and a woman in which he tells her to ask the girl about her dreams.
Both the prosecution and defense say they will not reveal Kasper's familial relationship to Gacy to the jury, calling it "irrelevant."
Gacy was convicted of killing 33 young men and boys but was suspected in numerous other cases. He was executed in 1994.