Usher's steamy serenade hit a cold note in Nashville ... prompting the R&B superstar to pull the plug after a woman onstage appeared less than thrilled by his up-close moves.

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Video from Saturday night's show captures a shirtless Usher leaning over the seated fan and trying to turn up the heat -- but her reserved reaction quickly catches his attention.

Moments later, he backs off and tells the crowd, "I don't think she wanna be onstage, y'all," bringing the intimate encounter to an abrupt end.

The awkward moment quickly made the rounds online, where viewers roasted the woman for seemingly fumbling a fantasy plenty would happily trade places to experience.

The woman -- identified as Gabrielle Cheyenne -- eventually fired back ... insisting "NOBODY" declined to go onstage and pointing out she had no problem letting thousands of people see her looking good on the venue's jumbotrons.

Gabrielle explained she and her mom originally had floor seats before Usher's team approached them and upgraded them to VIP ... adding she wasn’t told which performer she'd be joining onstage -- Usher is currently touring with Chris Brown.

She also mocked critics who apparently expected her to get much more physical with Usher in front of the entire arena.

In a separate post, Gabrielle argued her attitude only turns "nasty" when she's provoked ... saying she's quiet in real life but won't let strangers bully her over an experience they didn't get.

So, no, Gabrielle says she wasn't rejecting the spotlight -- she just wasn't ready to give Usher the reaction his fans wanted.