On the first day of the trial, David Mueller denies groping the star in 2013 – but Swift says she is “certain” she was assaulted.
The country and western DJ is suing Swift for at least $3m (£2.3m) in order to clear his name and recover lost earnings.
He was fired from his $150,000-a-year “dream job” at country station KYGO-FM after Swift’s team reported the alleged assault to his boss.
Swift is counter-suing Mueller for a symbolic $1 (77p) in compensation, and she says she wants to serve as an example to other women who have been assaulted.
Taylor Swift’s mother, Andrea, cried as she listened to opening statements in the Denver courtroom.
According to her lawyer, Swift is “absolutely certain” she was sexually assaulted – and he asked why the singer would ever make up such a claim.
Swift has said Mueller slipped his hand under her dress and grabbed her bare bottom as they posed. She was 23 at the time of the alleged assault.
Her lawyer also says Mr Mueller has changed his account of the incident seven times, and destroyed evidence relating to the assault.
Mueller’s lawyer showed the eight jurors the photograph which was taken at the time of the alleged assault.
Everyone in the picture is smiling, and Mueller’s right hand is hidden behind Swift.
He said the photo shows his client’s hand “is not underneath Miss Swift’s skirt, and her skirt is not rumpled in any fashion.”
The trial is expected to last for 10 days.
About 25 members of the public were in court to witness the hearing – with three teenage girls showing up at the courthouse before dawn to secure a spot inside.
“It’s better than a concert,” a 17-year-old girl said.