Unfortunately, it's not just the physical that lost her that match. Petra had more than enough chances, and was certainly in the drivers seat to close it out, and was actually winning most of the longer rallies.
Petra has many problems now. You can see her confidence, anticipation, instincts, serve, returns, groundstrokes, strategy, brains/strategy, etc., have all diminished or fell off this year as well, outside of anything physical.
However, it is correct to ask: "What the hell was Petra and her team doing during the off season!?"
Unfortunately (especially based on Petra's post game comments after Sydney, regarding her new Fitness coach), I think the end of Petra's season last year while ill/recovering (Playing Fed Cup, Exo and Czech Extraliga), cut into her rest and preparation time, as I been saying.