Las Vegas police have reopened a case of a sexual assault reported by Kathryn Mayorga.
In a statement, the police said: "The Las Vegas Metropolitan Police Department responded to a call of a sexual assault on June 13, 2009.
"At the time the report was taken, the victim did not provide detectives with the location of the incident or suspect description.
"A medical exam was conducted.
"As of September 2018, the case has been reopened and our detectives are following up on information being provided by the victim.
"This is an on-going investigation and no further details will be released at this time."
Ms Mayorga, 34, had told Der Spiegel that she was raped in June 2009 in a Las Vegas hotel room.
The magazine reported that her and Cristiano Ronaldo reached an out-of-court agreement which involved her remaining silent on the alleged rape and him paying her $375,000 (£275,000).
On Monday, the football star denied claims he raped Ms Mayorga - describing the allegation as "fake news".
In his Instagram Live video, Ronaldo answered a question from a fan and said: "No, no, no, no, no - what they said today [is] fake, fake news.
"They wanna promote by my name, it's normal.
"They wanna be famous to say my name, yeah, but it's part of the job.
"I'm a happy man and all good."
The police have not commented on or confirmed any possible involvement in the case by the footballer.