What many in this thread fails to realize is Serena's fight.
She can be down, injured, laboring in pain...she finds a way to win. This is what separates a champion from the rest. She did this when down with Cibulkova and now today, with Li Na. The only individual who has now become a more formidable threat that capitalizes on Serena's errors is Azarenka, but apparently she is also more prone to injury as of late.
As always, the series is Serena's to lose.