10/03/2011 at 10:10 AM EDT
Don't let the cowboy boots fool you.
stepped out of her pop-country niche at her Atlanta show Sunday night for a duet with rapper T.I., 31, to his 2008 hit, "Live Your Life."
Rocking black boots and a sparkly LBD, Swift, 21, performed the part of the hip-hop anthem originally sung by Rihanna
She later gushed on Twitter
about the "unreal" performance. "LOUDEST screams from the crowd, 14,000 people, hands in the air," she said.
T.I. (full name: Clifford Joseph Harris, Jr.) – who was released from prison
in August – wasn't Swift's only surprise guest in Atlanta. At a show the night before, the Grammy winner welcomed Usher
to the stage for a duet
of his 2004 hit single, "Yeah!"
Swift has also shared the stage with stars such as Justin Bieber
, former John Mayer
and Nicki Minaj.