Selena Gomez threw more than a few punches for her first workout after her kidney transplant, and "she killed it"
Selena Gomez on way to the Rumble GYM in New York City
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Selena Gomez is back in fighting shape!

The singer threw more than few punches at Rumble in New York City on Thursday for her first workout after her kidney transplant, and according Rumble co-founder and co-owner Noah D. Neiman, “she killed it.”

“She just came in and went hard. We were all, ‘Okay, that’s what I’m talking about!’ ” Neiman tells PEOPLE of Gomez, who took his colleague Joe Ferraro’s class.

EXCLUSIVE: Learn to box like Rocky, in Sly Stallone's new gym at Rumble in NYC
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But Gomez, 25, told them that she can go even harder.

“She said, ‘No guys, I’ll bring my A game next time’ and I was like, ‘What?! Look at you, you just had surgery,'” Neiman says. “She legit has a whole new kidney! But she was great.”

Selena Gomez seen coming out of rumble gym in NYC
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Gomez revealed on Sept. 14 that she had undergone a kidney transplant as part of her lupus treatment, and her donor was her best friend, actress Francia Raisa. A source close to Gomez told PEOPLE that the popstar is feeling better than she has in months.

RELATED VIDEO: Selena Gomez Reveals She Is Recovering From a Kidney Transplant Due to Lupus

“She’s really bounced back more quickly than everyone expected,” the source said of Gomez, who hit the gym in a Maje x Schott coat in between filming an untitled Woody Allen movie in New York City.

And Raisa, 29, is also hitting the gym after the transplant, posting on Sept. 22 that she’s “happy to be back” as she lifted weights.

Justin Bieber swaps nightlife for boxing, at super-trendy NYC gym.
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Rumble, which is known for their group fitness classes featuring 10 rounds of boxing and strength training, is a favorite of many celebs. Justin Bieber, Kendall Jenner, Teyana Taylor and Hailey Baldwin have all been spotted at the gym when they’re in the Big Apple.

  • Reporting by JANINE RUBENSTEIN