Serena is 9-0 against Top 5 players in 2012 and has only lost two matches since late March (and a mere four matches all year long despite playing a complete season for the first time since 2009).
Serena will face Sharapova, who played excellent tennis to finally stop her hard-court losing streak to Azarenka. The problem for Sharapova is that the last time she played Serena she barely won a game, and the fact is that she has not beaten Serena since the finals of the 2004 WTA Championships in Los Angeles (which I attended). She doesn't only lose to Serena, she loses badly. The recent matches have not even been very close (although the earlier meetings used to be). One really can't call theirs a rivalry because Sharapova has won so few of their matches over the years. This fact should free up Sharapova to go for broke because she literally has nothing to lose. No one expects her to win this match so she should hit the ball even harder than usual.
As for Serena, her problem may be over confidence. She is now 11-1 against World #1 Azarenka and 9-2 against World #2 Sharapova and in 2012 has won 7 consecutive matches against her putative rivals for the throne at the top of women's tennis. With a win on Sunday she will end the year with 58 wins and only 4 losses on the year.
That being said, Sharapova can be dangerous. However Serena looks like a woman on a mission and is determined to reach World #1 again. Serena has won a tour-leading 6 titles in 2012 (tied with Azarenka) and if she wins in Istanbul it will be her 7th title of the year in as many finals and her 3rd time winning the year-end championships (2001 walk over Lindsay Davenport and 2009 d. Venus Williams).
In fact, I would not be surprised if we start talking about a Serena slam sometime in 2013, especially if Serena wins the Australian Open, which is rather likely. She has a mere 3 more majors before she reaches the gaudy totals of the all-time greats Martina Navratilova and Chris Evert, which at the age of 31 is something she could very well reach in the not too distant future!