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P!nk's got the flu -- the nasty flu that's been spreading and intensifying this season -- and her illness could keep her from singing "The Star-Spangled Banner" on Super Bowl Sunday.

Sources close to Super Bowl LII production tell TMZ ... P!nk had a really tough time making it through her rehearsal Thursday night. We're told it didn't go smoothly ... and everyone involved didn't feel good when it was over.

There's more cause for concern too -- P!nk managed to power through her pre-Super Bowl concert Friday at The Armory in Minneapolis ... but had to rely to backup singers and the audience often to sing for her.

She even had to cut off her track "Beautiful Trauma" shortly after starting, telling the crowd, "I can't do it. I hate this ... I can't do that song. I'm sorry."

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The singer assured fans she's trying to fight through her illness and "practice the flu away," but as we know ... the flu's hit hard this year and takes a few days to recover ... even with rest. And it doesn't seem like P!nk's resting too much.

Still, she says she promises she's going to do her best.