Ok, if Seles was a nightmare for Graf, what did Seles have that Serena doesn't?
Serena has a tremendous serve, that few players can handle when she's serving well.
You don't hit winners using a backhand slice. Seles, Serena and great players like them could make you pay from either side, they were also able to go from defense to offense in one stroke from either side. I used to watch Graf a lot and she would always be standing closer to her backhand side of the court, looking for that big forehand, opening herself up to a player like Seles, who had powerful ground strokes. The great Chris Evert herself is calling Serena the greatest.