Lee's crushing defeat of martial arts master Wong Jack Man proves that he was a legit fighter and not just an entertainer.
http://en.wikipedia.org/wiki/Wong_Jack_Man
Lee got his start fighting on the streets of Hong Kong. There's little doubt that he was a real fighter and a great one at that. Best that ever lived? Not by a long shot but he was still a force to be reckoned with and did a great deal for the world of martial arts.