Sharapova not asked/didn't answer/stormed off/had her publicist intercept the question prior to the interview.
Or she gave the polite/boring/PR answer that wasn't as interesting as what the others had to say. More to do with the program's time limit, I would guess. Don't think Li's answer was in there either.
Edit: In response to the sore loser comment(s) above and below, none of us know what goes on in Maria's head. She'd answer the questions as she can and has been honest in the past about the lack of rivalry, but if you're expecting her to come out and say she sucks and will never beat Serena again, don't hold your breath.
Last edited by BadgerM; Nov 27th, 2012 at 08:37 PM.
To answer the question, to challenge Serena you need to be a good mover like Clijsters, Henin, Venus, Capriati, JJ, or even Hingis whom despite her physical limitations was a great mover. They all forced Serena to play more than one great shot and could come up with good shots on their own. But at the end of the day, Serena at their best was just way better than them.