Tonya Harding Most Brutal Knockout Video ... Unearthed

Wanna see Tonya Harding knock a chick out cold with a devastating right cross?

You're in luck ... because TMZ Sports found extremely rare footage from Tonya's last professional boxing match -- and she straight DESTROYED her opponent.

It all went down in 2005 -- when a 35-year-old Harding (3-3) stepped into the ring to face Brittney Drake in Brittney's first pro boxing match, a fight promoted by Celebrity Boxing creator Damon Feldman.

Makes ya wonder how good Tonya coulda been if she trained to be a fighter instead of a skater ...