Swiss star Federer, 27, will overtake Sampras' record of 14 Grand Slams if he beats Andy Roddick in today's Wimbledon final.
But seven-time SW19 champ Pistol Pete insists Federer is already the best player to have graced the courts.
Sampras, 37, said: "What he's done over the past five years has never, ever been done and probably will never, ever happen again.
"Regardless if he beats Roddick, he goes down as the greatest ever. This just confirms it.
"I'm happy for Roger. If there's anyone that deserves it, it's him. He is a great player who is a credit to the sport and a positive influence for young kids.
"It looks tough to beat now with 14 Majors and I'm sure he'll win more."
Federer completed a career Slam, when he won this year's French Open, the only Major that had eluded him.
And eight-time Slam winner Andre Agassi, the last man to complete a haul of all four Majors, said: "How do you argue with his numbers?
"It's incredible. A lot of people say it's better to be lucky than good. I'd rather be Roger than lucky."
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