Britney Spears was one-degree away from reuniting with Justin Timberlake last night -- after bringing one of his *NSYNC bandmates on stage during her new Las Vegas show.

As part of her act, the singer selects one male "hottie" from the audience to join her on stage, only to be whipped and led around like a dog on a leash during her "Freakshow" performance.

Last night, that lucky audience member was Lance Bass.

He definitely seemed to be enjoying himself up there, where he was seen sporting kitty ears and an S&M harness on his chest.

On opening night of her "Piece of Me" residency, Mario Lopez won the honor -- and was also more than game to ham it up for Ms. Britney Jean. Check out the hilarious video below!

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