I don’t know if checkers is a “sport” but I find the record of Marion Tinsley to be beyond astonishing. The Wiki article states he lost only 7 games in his 45-year career. He was so invincible he stopped bothering to enter tournaments.
By now, computers can play perfect checkers, but what I find singularly impressive is that for a few years the Chinook checkers program was apparently able to beat every single human on the planet … except Marion Tinsley.