I will go with Petra in 3. It isn't that Petra played badly today against Serena more along the lines Serena just was even better at everything today. She has enough fire power to keep Angie off balance.
Maybe this is the case of who has recovered from her Serena spanking. I am going out on a limb and say Angie's win over Aga has injected her with some belief, and pushed her past her loss to Serena. Where as Petra's beat-down, still has her down. Which Petra will show up
Petra in straights. Petra was playing quite well yesterday. Of course Serena was better, but Petra's body language was OK. I believe Petra can recover very quickly as she's proved in many trhreesetters already.
Their last battle in Tokyo was a 3-set bloodbath. Where Petra ate her random bagel and then found her serve again. I think Angie will be extremely motivated for payback. Kerber in 3.