Former U.S. Olympian Jamie Nieto is kicking rehab's ass ... walking with only a walker just 5 1/2 months after sustaining a life-threatening back injury.
As we previously reported, Nieto -- a world class high jumper -- landed on his head while training ... and suffered severe damage to his back. He was unable to move and required emergency surgery.
TMZ Sports spoke with Nieto's manager, Paul Doyle, who tells us he's "getting better physically "and is able to stand and walk with only a walker.
"He's had tons of friends visiting him and helping out. A friend from Sweden even flew in to see him. It's been amazing."
Jamie's made remarkable progress ... check out the video.