Swift, 27, will testify, says Reuters, that Mueller slipped his hand under her dress and grabbed her bare bottom as they posed during a meet-and-greet session before her June 2, 2013, concert in Denver.
In a deposition, Swift said: "It was not an accident, it was completely intentional, and I have never been so sure of anything in my life". He alleges that the time of the incident, he asked that the police be called.
According to pre-trial legal documents obtained by CNN, Mueller launched a law suit in 2015 claiming Swift and her co-defendents (including her mother), falsely accused him of improperly touching the star, and is now seeking £2.3 million in damages.
Mueller is also expected to testify, along with Mueller's former boss and members of Swift's entourage.
Swift tried to keep the situation "discreet and quiet and confidential" and was upset by Mueller's claim that "for some reason she might have some incentive to actually fabricate this story", her attorney, Douglas Baldridge, has argued in court. He says it wasn't him.
Swift also will be required to be in court for every day of the proceedings, except during jury selection.
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Mueller said Swift was cordial as he and his girlfriend left.
How did David Mueller respond?
Ms. Swift's pretrial deposition provided a preview of her testimony about the events at Pepsi Center, the arena in Denver not far from the courthouse.
According to TMZ, 32 seats will be open to the public for those interested in the trial (the case has predictably drummed up plenty of buzz among Swift's fans and locals).
The DJ says as many as 20 other people could have been in the room at the time and insists the security guard present did not react to anything during the meet and greet.
Mueller denies anything inappropriate occurred during the brief backstage encounter in which he stood on one side of the pop star and his girlfriend on the other.
After today's jury selection, the AP reports opening statements should begin tomorrow in the case which could last two weeks. The trial is expected to last nine days; likely until August 17. A member of Swift's security team also told Mueller's boss at KYGO radio station what happened, and Mueller was sacked, according to a lawsuit he filed.
Mueller's lawyer, Gabriel McFarland, has argued in previous hearings that he could be the victim of mistaken identity.