They do if they played a 3 setter in the Australian heat and might be nursing an injury and playing on adrenaline. However, I am reminded of a 17 year old banshee who did manage to take Serena out in embarrassing fashion at one time, so anything is possible.
I am not saying Keys can't win it, but Serena seems to have dialed herself in now that she is now in the semis. Taking Serena out this late in a slam is a tough task. Also, taking out two Williams sisters is not an easy ask either.