DENVER: Music superstar Taylor Swift told a US court Thursday of her shock as she realised she was being groped during a photo opportunity in Colorado, as she faced the former DJ she accuses of assaulting her.

Swift, 27, said there was no doubt in her mind the radio personality she was being photographed with ahead of a Denver concert in 2013 reached under her skirt and fondled her buttocks.

"It was a definite grab... a very long grab," Swift testified in a Denver federal court. She declined to speculate how long the incident lasted.

"He stayed latched on to my bare ass cheek as I lurched away from him. (His hand) didn't let go," she told the jury.

Swift was called as a witness by the attorney representing David Mueller, the former Denver radio host accused of groping her during the photo session.

"It was a very shocking thing. I had never dealt with something like this before," Swift said.

Mueller, 55, is suing Swift, alleging she and her team slandered him by telling his employer about the incident, resulting in his firing.

She is counter-suing, alleging she was sexually assaulted.

"Someone could have concluded it appeared she was trying to get away from me," Mueller testified Wednesday when shown a picture taken during the alleged assault.