Serena will win.
This same situation happened to Lindsay Davenport at the 2001 Pilot Pen tournament when she was supposed to play Kim and Kim withdrew with a thigh injury. Venus had to play two matches the day before the final because of rain. That meant Lindsay was rested (having not played since that Wednedsay) and was favored to win.
Venus won in two sets. That is why Serena has a better chance at winning because she is more match ready that Kim, like Venus was against Lindsay.