Taylor Swift jams with Nelly at birthday bash

Taylor Swift surprised partygoers in New York on Saturday night (06Aug16) by joining rapper Nelly onstage to perform his hit 2002 song Dilemma.

The Shake It Off hitmaker is currently enjoying some downtime after a busy 2015, but she couldn't resist picking up the microphone on Saturday as she partied with oil heir Mike Hess in the Hamptons for his birthday.

In video footage posted online, Taylor tackles Kelly Rowland's parts in the Nelly duet as the pair dance together, much to the crowd's delight. The stars previously collaborated last year (15), when Nelly served as a special guest on Taylor's 1989 World Tour when it stopped in his hometown of St. Louis, Missouri.

Taylor's boyfriend, actor Tom Hiddleston, was not present for the Hamptons bash, but she appeared to have a blast regardless, and was also caught on camera dancing with birthday boy Hess and planting a kiss on his cheek during her night out in the Hamptons.

"Taylor was in an amazing mood, and super fun all night," a fellow partygoer tells E! News. "She danced her a** off. At first she didn't want to perform but then Nelly asked her. Everyone was super into it, the crowd loved it."

The 26-year-old, who wore a black crop top and mini skirt for the occasion, attended the party with her close pal Karlie Kloss, who was also celebrating after turning 24 on Wednesday (03Aug16).

Other guests included former supermodel Heidi Klum and Broadway star Leslie Odom, Jr., according to the outlet.