Man o' War's only loss came on August 13, 1919, in the Sanford Memorial Stakes, when Upset defeated him despite Man o' War having beaten Upset in six other races. The result has been attributed partly to circumstances and partly to the shrewd riding of Upset's jockey, Willie Knapp. Knapp recalled that in the seven-horse race, with Man o' War favored at 11 to 20 odds, Golden Broom set the pace while Upset ran just outside. As Man o' War advanced along the rail on the turn, Knapp refused to leave room on the inside, knowing that if Johnny Loftus could come through, the race would likely be over. When Loftus finally pulled Man o' War sharply to the outside, Knapp drove Upset forward and gained a decisive advantage, forcing Man o' War to swing around both horses and lose two lengths. Man o' War charged again near the finish, but Upset held on by a head. Knapp called it the greatest thrill of his life, yet he also said Man o' War was a colt unlike any other and believed he should have retired undefeated.