Good Points Bruce.
Petra is probably the only woman on tour that Can Beat Serena (all thing being equal, of course Serena could lose to Azarenka Kerber or God Forbid--even Masha on a bad day LOL) if she was mentally prepared and was playing her A game.
Many times, Serena doesn't play great. However her opponents are usually ill prepared to take advantage, cause they don't go for the jugular. Usually it's the scrubs that actually go for the jugular against her (like Makarova at the Australian and Razzano at the French Open) when they have Serena on the ropes.
Now of course it appears, Serena tries/plays harder against the likes of Azarenka, Sharapova and Kvitova, then Radwanska, Kerber and Jie Zheng.
Nonetheless, Petra has the game (Serve, Return, Heavy Groundies, Net Game, Slice, overall shot making, etc.) to beat Serena. The real question is, will she ever be at a high enough mental, physical, game and strategic level to do so?