Venus has really had a fantastic tournament and has a lot to be proud of. She has not allowed her debilitating illness to stand in the way of her playing top level tennis.
Petra also had to play very well to beat Venus, it was very good tennis and a long match. This may be factor when Petra plays Angelique Kerber in the final.
Angelique beat Caroline Wozniacki in straight sets. so Angie might have more energy in the final than Petra.
Petra played two matches yesterday and a long two and half hour match against Venus today.
But in tennis anything can happen and I am sure Petra will get a adrenalin rush from making it to the final.
Angelique is the higher ranked player but I would not rule Petra out from winning this title.