Taylor Swift Parties in the Hamptons at Bestie Karlie Kloss' Birthday Bash, Performs With Nelly

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Dilemma or just good fun? Taylor Swift rocked out over the weekend at a birthday party for her bestie Karlie Kloss and oil heir pal Mike Hess in the Hamptons. The “Blank Space” singer joined rapper Nelly on stage at the soiree, singing Kelly Rowland’s part to his hit song, “Dilemma.”

An eyewitness tells ET that the 26-year-old GRAMMY winner was having fun at the party when Nelly asked her to join him on stage.

MORE: Karlie Kloss Gets Love and Long Leg Praise From Taylor Swift, Eva Longoria -- See the Sweet Birthday Wishes!

“She hesitated at first but decided in the end to do it,” the source tells ET.

Swift, wearing a black off-the-shoulder crop top and matching skirt, danced along to the popular tune, and was captured on Snapchat goofing off with Hess.   

Another source tells ET that there was nothing at all romantic between the pair and that Swift gave the birthday boy a sweet peck on the cheek after wishing him a happy birthday. 

The singer’s new beau, Tom Hiddleston, was not at the bash, the source confirms to ET.

In addition to Swift, Nelly, and the birthday girl, other celeb guests included Chris Rock, Hamilton alum Leslie Odom Jr., and supermodel Heidi Klum.

Swift and Hiddleston were last spotted together earlier this month. The Avengers star left Swift’s Los Angeles home to return to Australia solo, where he is filming a movie.

WATCH: Taylor Swift Sports a Sexy Velvet LBD for Date Night With Tom Hiddleston -- See the Hot Look!

Though the singer has been noticeably quiet on social media since her blowout feud with Kanye West and Kim Kardashian, she did take a moment to celebrate Kloss’ birthday on Instagram.

“I LOVE YOU KARLIE! You're such an exquisite person, always trying to make things better for others. Every day I'm inspired by how giving, loving, and thoughtful you are. Happy 24th Birthday!!!
@karliekloss,” she captioned a cute selfie.

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