Swift, 23, kicked off the ceremony in Los Angeles with a rendition of her hit "We Are Never, Ever Getting Back Together."
"So he calls me up and he's like, 'I still love you,'" Swift sang in her faux British dialect.
"And I'm like, 'I'm sorry, I'm busy opening up the Grammys,'" Swift sang. "And we're never getting back together. Like, ever."
Swift and Styles, who just turned 19 this month, dated from October 2012 until last month.