Sampras also missed AO a couple of times and the year Tim fell sick, he couldn't win AO because he was too upset about it, though he later dedicated that win against Courier to Tim. Federer, on the other hand, I don't think ever missed a Slam either due to injuries or for personal reasons. He also has longer career than Sampras. He doesn't have the record as holding the No. 1 ranking for 6 straight years which is a gruesome deal itself and can take a lot of a player. Also, physically Sampras was a bit more compromised as he had a condition in his blood that wasn't ideal for slower surface and longer rallies.