OH no, Maria still has a mathematical chance:
"If Serena goes 1-2 in round robin play, then Sharapova needs to reach the final with 2-1 round robin record or win the title to have a chance."
Both of the above conditions are mathematically possible.
But, for all practical purposes it is over.
However, it is hard to imagine Serena losing both of her remaining RR matches. If you are
willing to give Serena one more RR match win, then the only way Maria can become YE #1 is if
she wins the whole thing as per the following statement.
"If Serena goes 2-1 in round robin play, then Sharapova needs to reach the final with 3-0 round robin record or win the title to have a chance".