ScentOfDefeat
G.O.A.T.
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.
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.