In my opinion, I still feel like Sharapova will win the tournament.
Venus has indeed struggled with Radwanska in the recent years, actually she has her hands full with Giorgi next round. Ever since she's been diagnosed with Sjogren's, big hitters + the best defenders on tour (besides Wozniacki and Azarenka) have given her trouble. The hitters are probably giving her trouble because she doesn't have the movement she had a decade ago. However as for Aga, she has to deal with Lepchenko, who is on a two match win streak against her. It's not guaranteed either player will make round 4
I've seen Serena play, and she's sloppy. Her forehand is okay but her backhand is very off, no footwork (as usual), and while her serve works in patches it's not up to par yet. She will improve if she stays healthy, or better yet she has to. Muguruza will eat her alive if she plays like she did in the first two rounds (she should have no trouble with Svitolina tomorrow).
Kvitova is very hit and miss. She played poorly in Shenzen, spectacular in Sydney. Who knows where she's at. However if she keeps it together the only person in my opinion who can beat her is Serena, and even then Serena will need to be 100% or extremely close to that beat her. Dubai 2013 comes to mind
Sharapova indeed struggled, even saved some MPs, but I have to bet on her because even when she's off she fights to the end. Serena does too but there are too many landmines on her side of the draw for her to be off and try to come back. Sharapova was lucky Panova got nervous.
Besides Serena, Kvitova, and Sharapova, everyone else has outside chances (Bouchard, Halep, Radwanska, Vika, and even Venus given her 2015 play) but I wouldn't beat on them.
go back to marathoning and leave it to professionals
even if for a short period of time