It's always about "star-power", not about official WTA rating. BTW, home players (aka crowd favorites) even if not ranked in TOP50 always has chances to play at Central Court. Look example of #180+ ranked Dokic at 2009 AO which played last three or four matches at Rod Laver Arena, playing just once against TOP10 opponent (which means SHE is reason).
At Roland Garros 2009 probably, and maybe at Wimbledon 2009 also, 1st seed will be Dinara Safina, but for both organizers (b/c "star-power") she will be still behind Sharapova (or Sisters, or even Ivanovic) from WTA part. Star-power sell the tickets and that's how things works in tennis.
and I'm not even fan of Ana Ivanovic... just an objective observer who realize how perfect she is
AMAZING BELGRADE MIRACLE: In just three years (2008-2011) 5 different players born in same city reach world #1 in rankings and several GS titles and finals (WTA singles: Ana Ivanovic and Jelena Jankovic, ATP singles: Novak Djokovic, ATP doubles: Nenad Zimonjic and Daniel Nestor... with notable performances of Bojana Jovanovski, Janko Tipsarevic and Viktor Troicki which is also all three born in Belgrade)