Some people are just putting down players they don't like.
Unless it's one of my absolute favorites (Myskina, Dementieva, Clijsters, Molik) then I usually cheer for the underdog. That doesn't neccessarily mean who is ranked lower, as I'd consider Petrova an underdog to Serena anyday.
Riding the 3AM Rollercoaster!! Players I support, no matter what
Anastasia Myskina, Kim Clijsters, Alicia Molik, Elena Dementieva, Svetlana Kuznetsova, Tatiana Golovin, Ai Sugiyama
Juan Carlos Ferrero, Lleyton Hewitt, Tommy Robredo, Rafael Nadal, Gaston Gaudio, Nikolay Davydenko, Richard Gasquet, Jarkko Nieminen, Mario Ancic, Mikhail Youznhy, Novak Djokovic, Marin Cilic, Andy Murray, Evgeny Korolev, Paul-Henri Mathieu, Dmitry Tursunov, Stanislas Wawrinka