Never beaten a good Nasus. He eventually just ignores you until he feels like going for the kill. You've got until his first B to win lane.
God help you if it's a Ghost Dog.
Pretty much this. As Nasus, once I get a shroud its GG against a Jayce. I run 9/0/21 with full tank rune page. 3% run speed+wither+ghost means a Jayce can hammer me away all he wants, I'll still be on top of him with an ult just bonking the crap out of him.