Haha. Exactly. If you're fatigued you're fatigued. Call in. It's the professional thing to do.

Thing with my company is, (maybe others/all?), they don't want to hear "I think im going to be fatigued if I do this flight". Either you are fit for duty or you are not. This has bit me in the ass before when I found myself having no business being in the flight deck 1/2 way through a 2 hour plus leg.
So, when I call in now, it is "I AM fatigued" and I am removed from duty.