Yes, well I'm aware of that. In boxing the champions are also vulnerable, as they eventually lose (except for a few exceptional ones). The only reason why I think boxing doesn't work as a tournament sport is that each fight is very taxing for the boxer and they could become permanently injured if they had to win 7 fights in a row to lift a trophy. Tennis fans are eager to watch tennis week-in-week-out, even if it means watching very poor matches up until the final.You play against the field in tennis. As humorous as I found your comment about turning the WTA into a "challenge Serena" system, even Serena is not invulnerable. She did lose fair and square at Wimbledon this year.