A fatigued flight crew failed to modify their FMS routing when their ATC clearance was modified from their originally filed route.

Narrative

We were preparing to operate our last flight of a 3 day trip. We could not get a PDC from RDU; so I contracted clearance delivery to pick it up. They issued: Pack6 AZELL direct SPA Falcon3. This was a change from our filed routing of: Pack6 AZELL direct CAE J4 IRQ Canuk7. Neither I or the Captain noticed the change and the original filed clearance was already entered in the FMS by the time I received the change. We did verify the legs prior to departure; but used the original filed route for verification. When we asked for a short cut; our controller realized we must not have the correct routing. He inquired as to what filing we had and the problem was corrected before it could create an actual deviation. Fatigue and distractions played a primary role in this error. This was the last leg of the day with around 14 hours on duty. This was a bad weather day; resulting in long ground delays. We spent much of our short ground time negotiating with ATC and were successful in getting a very long ground delay reduced. There had been concerns we would time out. I believe we realized we were quite were quite tired by this time. We were trying to check behind each other to make sure nothing was missed and yet; The FMS was updated to reflect the correct routing.