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Kendall Jenner 's making it hard to keep up with her these days ... and, we mean that literally 'cause she was zooming around the water at a lake in a tiny bikini!

The star was spotted in Washington state earlier this week ... cutting up the bright blue water in a little brown bikini, which showed off a lot of her popular posterior. Backgrid

Jenner's locked in here ... keeping her concentration while zooming around the lake on the stand-up jet ski -- no stunt shots of her grinning for the cams here. Backgrid

KJ's got perfect balance here ... kneeling down at times and bending forward at others -- and, we're surprised no one crashed around her, since she usually draws every eye in the area.

It's some much-needed time off for the star ... wh

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