EXCLUSIVE: Jimmy Fallon & Justin Timberlake Enjoy Epic Night Out With Kate Hudson and Jennifer Lopez

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This is what it's like when celebrities go hard.

Jimmy Fallon and Justin Timberlake continued their epic night in NYC on Wednesday along with a cavalcade of celebrities, including Jennifer Lopez, Casper Smart, and Kate Hudson, who wore a SUPER low-cut jumpsuit that appeared to land the actress in a close call with a wardrobe malfunction.

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An eyewitness tells ET that after Justin's Tonight Show return, followed by his show-stealing U.S. Open appearance with Jimmy, the two were joined by J.Lo, Casper, and Kate at the 40/40 club, before the group -- sans Jimmy -- moved on to the Game nightclub.

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While Jimmy dipped after 40/40, Justin partied on at Game until after 4 a.m.! We're told his drink of choice was Tequila. (His Sauza 901 brand, perhaps?)

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"Everyone was so happy," the eyewitness reveals. "Casper had a talk with his old driver who now drives Timberlake. Jimmy is always all smiles."

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Earlier in the night, J.Lo, Casper, and Kate stopped off at Nick Jonas's concert, where Hilary Duff, Hailee Steinfeld, Gigi Hadid, and Joe Jonas were also in attendance.

Judging from Kate's "current mood" picture posted to her Instagram the next morning, the night got pretty wild.

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After taping the Tonight Show, Jimmy and Justin watched Roger Federer’s quarterfinal match against Richard Gasquet from the Emirates Airline suite, where they launched into an impromptu Beyonce "Single Ladies" dance routine.

All this transpired from an evening that began with Jimmy sharing pics of Justin and Jessica Biel's ADORABLE 5-month-old baby boy, Silas.

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So, don't ever say Jimmy and Justin don't know how to turn up! 'Cause those BFFs will prove you wrong.

Watch the video below.