I've never seen anyone claim they were the first to turn defence into attack, every great since Court has been able to do that to some extent. Show me an example of a commentator claiming no-one had done this before.
Read the thread about Agassi's comments.
Obviously where the Williams sisters revolutionised the game was by hitting hard off both flanks (Davenport, Seles, Pierce) combined with athleticism (Graf) combined with record breaking and nigh unbreakable serves (Navratilova). No-one
before them had brought all these things together into one package, no-one has since. The bar is still standing where Serena and Venus set it in 2001-2002.
As was said, people watching the game know when the bar has been raised
. You've got nothing
Graf had world-class power and athleticism, and to a higher degree seeing as she actually translated her athleticism into world-class DEFENCE, unlike the Williams sisters who have never been capable of winning matches on their defensive skills alone like Graf could.
So basically, again, all you've got that the Williams sisters could do that others couldn't is serve.
IF they didnt revolutionize the game than why is Serena, more so now than Venus, still winning tournaments on the tour and thumping almost all of the top players consistently? If they didn't raise the bar I don't really know how we can say anyone has revolutionized the game by your logic?
You bring up the fact that their serves have been better than anyone before them like that is something to slouch at? Are you freaking kidding me dude?? You have to really gain some perspective if you seriously think all they do better than their predecessors is serve. Even if that were true, holding serve is the most fundamental method to win in tennis, THAT alone revolutionized the game. If Graf or Navratilova had Serena's serve they would have way more titles than they have even now.
Please read what I said. I accepted in principle that the Williams sisters advanced
the game in that they imported men's tennis-style serving, but I said that on that basis, then Stosur has also advanced the game by bringing ATP-style topspin into the game (and thus, advancing the game and bringing in something that hasn't been done before =/= being better, unless you consider Stosur to be better than Serena).