"You just blast it in your room and sing in front of your mirror into a hairbrush. You had that one favourite song that was your favourite to dance to. Los Angeles, this is my hairbrush song," she added. Lopez then appeared on stage to huge cheers from the audience before singing the 2002 hit.
Swift duetted with Ellie Goulding and Cher Lloyd during her other shows at the Staples Centre last week.