Andy Murray has entered the debate over the value of the tennis world rankings by questioning the Williams sisters' interest in tournaments outside the grand slams.
The rise of Russian Dinara Safina to number one has raised the question of whether it is justifiable for a player without a grand slam title to be on top.
Murray said: “People are saying, 'How can she be number one if she's never won a slam?'.
“It's because the ranking system rewards consistency, and there has to be an element of that.
“Do you not agree that it's important for the top players like Serena to turn up at the other big events.
“Tennis has an 11-month calendar. If you have ranking points which are too small for Serena to turn up at some of the smaller events, then all of a sudden the whole calendar is completely pointless.”
Yesterday, women's fourth seed Elena Dementieva and former World No.1 Jelena Janknovic both crashed out.
Dementieva went out 5-7, 6-4, 6-3 to American teenager Melanie Oudin, while Jankovic lost to Yaraslova Shedova in the match of the tournament, 6-3, 6-7, 7-6.