When it comes to athletic events, I do not root for an underdog just for the sake of doing so. An underdog is an underdog for a good reason -- he, she, or it has compiled a record that is not as good as that of the athletes or teams he, she, or it is up against. I may root for an underdog because the underdog is (for some other reason) an athlete or a team that I favor. But that's the end of it.
I prefer enduring excellence. That is why, for example, I enjoy watching Tiger Woods play golf. It will be a sad day for me when his skills diminsh to the point where he is no longer the golfer that he has been for most of the past ten years. I just hope that he is succeeded by another electrifying talent, not by a "committee" of also-rans.