Ok, the williams sisters made their mark on the tour, they were probably the ones who generally initiated the hard-hitting style of play. At least they're most known for that. They're both great players and they will always be that. But you have to face it, it's not anymore like 10 years ago, when one player could dominate the sport for several years. People got familiar with that type of play and adjusted their own games.
And it was about time for the 'next big thing', it came with Sharapova. Last year it were the belgians that made noise, now it's Maria. Before that it were the Williamses but they're not the only ones anymore.