It's almost perfectly symmetrical.
About a 2.5 year gap between his 1st and 2nd slams, then eight years
of at least one slam per year. Then a 2 year gap between those and his
last slam in 2002 when he was 31.
Now have a look at Federer's singles slam timeline.
Once again we have near perfect symmetry.
Eight years of at least one slam per year.
No gaps between his first slam year and his next slam year.
I believe this U.S. Open will be critical for Federer in two ways.
If he can win it, he keeps his string of one slam per year alive,
extending it to a record nine years.
If he does not win the U.S. Open this year, he will have broken his
one slam per year streak + it will have been two years since his last
slam win (A.O. 2010).
Sampras did manage to scrape by with another slam after his two year
slam hiatus, but was quite lucky to get that win.
Fed would find himself in the same situation Sampras was in if he
doesn't win this U.S. Open.
The good news for Federer is that he is 'just 30' and not 31 as
Sampras was. :)