Well, let's see. The two of them beat Hingis 16 times. But she beat them 16 times, too, so I assume she "out played" them on at least one or two of those occasions.
Truth is, the Williams sisters have a hard time acknowledging that anyone ever outplays them. But that's kind of a delusional idea that some players have about tennis.
It's like some player with a great forehand thinking, "I have a much better forehand than she does and if she just gives me some hard, flat ones to my forehand, I'm gonna killer her."
Well, that player is correct, except her opponent knows that, so she doesn't hit to her forehand, she hits to her crappy backhand and she wins.
See, its not your opponent's job to let you play your game, its their job to take you out of your game. And unless you are playing against a backboard, there is always the chance that will happen.
And guess what? When it happens, you have been outplayed, like it or not.