I've had stomach viruses. They suck and you feel like death for a while. But you certainly don't get one and think, "well golly, I don't think I'll be back in shape over two weeks from now."
Something fishy is going on here. Call me a conspiracy theorist, but I think the knee is in worse shape than we've been led on to believe and Nadal is trying to mix up his excuses to give fans hope and keep opponents on guard.