The Associated Press isn't identifying her because she's an alleged victim of a sexual crime.
She took the stand Tuesday shortly after a jury was seated and testified that details were unclear about the night of the alleged assault over the 2010 Labor Day weekend in Cox's apartment. She reported the crime to police after she learned she was pregnant.
Cox has pleaded not guilty to one count of sexual assault while the victim was physically helpless and one count of sexual assault while the victim was incapable of determining the nature of the conduct. He faces two years to life in prison if convicted.
Douglas County District Court Judge Paul King told potential jurors Tuesday that witnesses might also include Broncos wide receiver Demaryius Thomas, linebacker Wesley Woodyard, special teams specialist Cassius Vaughn - Cox's roommate at the time who said he was asleep in the apartment that night - as well as several doctors.
A witness list of those who are expected to testify in Castle Rock, 30 miles south of Denver, remains sealed.
Thomas, who was at the courthouse Wednesday, caught an 80-yard touchdown to beat the Pittsburgh Steelers in the playoffs last month.
Cox defense attorney Harvey Steinberg, in his opening statement, disputed prosecutors' version of events, which includes allegations that Cox impregnated the woman. Police arrested Cox after his DNA matched a sample taken from her.
Steinberg said Cox was out with his girlfriend that night, and that the victim was interested in Thomas.
"At no time did Mr. Cox show any interest in her romantically," Steinberg said.
"The DNA is not the entire picture."
The woman testified that she had no recollection that she had been sexually assaulted in Cox's apartment. But she told jurors that she suspected she had been drugged because she has a high tolerance for alcohol and did not drink that much, yet she doesn't recall many details of the evening, including walking up the stairs to Cox's apartment.
The Post reported that she testified remembering "fooling around" with Thomas, who was at a nightclub with Cox and his girlfriend and went to Cox's apartment that night.
"When I woke up, I felt funny," she told jurors assembled Tuesday in Douglas County District Court.
She contacted authorities on Oct. 28, 2010, after learning she was pregnant. Cox, Thomas and Vaughn all submitted DNA samples. Thomas and Vaughn were cleared.