This isn't exactly true. Nadal had to beat Fed in the semi at RG in 2005,just to get to the final. **** had to beat Fed in semi at the AO in 2008 just to make the final there,as well. Fed is the only one who did not have to beat a slam champ on his way to his first slam win.
I know those wins weren't in finals,but they still count as having to get through a proven champ to win their first slam,and not just any proven champ,but the GOAT.