Etymologically, a champion is someone who fought in the campus (hence "campaign"), which could mean a battlefield or an arena for staged battles. The participants were known in medieval Latin as campiones, which passed into Old English as "champion".
This sense (the "king's champion") survives in the idea of, for example, a champion of human rights, while the notion of "winner" developed last century. The Dictionary of National Biography (1840) talked about boxing's "champion of England", while Sharpe's History of Egypt (1860) referred to "The case of the champion fighting cock". And in the Saturday Review (1879), "Gold medals were won by the champion and championess."