A 10-1 fizz that can take on a Talon and a Darius can just roam and pick off kills by warding their jungle, split pushing (as no one can 1v1 you, and you got sick escapes) and winning through this. Teamfighting is not where Fizz shines.
But I know your pain. I had a 12-3-10 Diana game with a good Xin jungler and good Janna support. But if your AD doesn't do damage (Jayce?) and ignores everything but farm, just like your top (Nasus) you will eventually start losing 3v5. That was painfull...