If he only gets to the semifinals of Roland Garros and wins a Master without doing anything else, he will end the year ranked at #20. I doubt he will be doing that much to start with, so him ending 2013 at #20 is very plausible.
Unless he retires from the game permanently, there is NO way that is going to happen. NO way. It is wishful thinking on your part and I say this as a Federer fan.