Oh cool, nobody seems to have mentioned it, but on account of winning last night, Maria has secured YE#2, regardless of what Serena does in this event/to her.
Also, I believe the door could potentially be open for her to snatch #1, contrary to what others have said.
To do so, Maria would need to win the tournament unbeaten, which would give her 1500 points. In addition, she needs Li to eliminate Azarenka tomorrow, meaning Maria would claim #1 by the narrowest of margins. It won't happen, but it's still there I think!