Justin Bieber Busts His Ankle Hours Before Concert

Justin Bieber's basketball diary entry for today -- OUCH!!!!!!

Biebs went down hard during a pick-up game Friday in Cincinnati ... and came out of it with a severely swollen ankle.

He says it's sprained, which means 18,000 or so Beliebers are collectively holding their breath since Justin's supposed to perform tonight at U.S. Bank Arena.

They can all suck air again. Justin says he's going to have to take it easy onstage, but promises he's "still gonna crush."