Petra went down pretty easily, I will have to watch that match.
I saw enough of it to see why Bertens won.
Combination of a few factors.
First Bertens was out-serving Kvitova, with powerful serves that set up many points.
Second, Kvitova was tentative and off-form today, making too many unforced errors.
She didn't have a strategy really.
It was Bertens who was moving Kvitova around the court most of the time.
To beat Bertens your game has to be on (including serve) and you have to hit aggressively
all the time. Mixing things up of course too.