Video of the pop singer getting destroyed by an amateur's body shot has just surfaced ... and, yeah, somewhere Odom's got to be smiling.

TMZ Sports has learned it all went down on May 4 ... when Carter was in a Santa Monica gym prepping for his June 11 Celebrity Boxing tilt with the ex-NBA star.

We're told Carter was mixing it up with Ian Feiner -- a fellow gym-goer who he had sparred with several times before -- when he got caught by a right hook to the gut.

The video shows Carter didn't exactly handle it well ... he crumpled to his knees in pain and the session ended just a few moments later.

We reached out to Carter for his side of the story, but so far, no word back yet.

Feiner tells us Carter was "fine overall" ... but it's clear, even though this was over two weeks ago, AC's going to need some work to avoid a loss to Odom.

You'll recall ... Odom's looking GOOD in his recent training sessions -- and was already the heavy favorite heading into the fight.