Here is the current situation in the Red Group with only one match left to be played.
1. If Halep wins the match against Ana (independent of in how many sets),
Halep wins the Red Group and Serena will be the runner up (as Halep
will have 3 wins, Serena 2 wins, Ana 1 win and Genie 0 wins).
2. If Ana wins the match in three sets, Halep wins the Red Group and Serena
will be runner up (as Serena, Halep and Ana all will have 2 wins each and
Halep has 5 sets won out of 7, Serena 4 out of 6 and Ana 4 out of 7).
3. If Ana wins the match in three sets, we have an interesting situation.
Serena, Halep and Ana all will have 2 wins each. Further, all would
have won 4 sets out of 6 and so % of sets won will also be tied.
So, it will come to percentage of games won. Serena has won
52.17% of games and is done. We will know the % of games won
by Ana and Halep after the last match. The two girls with the
highest % of games won will advance to SF. While theoretically,
Serena still has a chance even in this scenario, her prospects
are not good in this scenario.