Sampras saw Federer go past his own 14 Grand Slams, and Nadal will match the American's total if he beats Switzerland's Stanislas Wawrinka in Sunday's Australian Open final.
"He's incredible," Sampras said in a BBC interview after witnessing Nadal in action for the first time live on Friday.
"The movement he has is incredible, he is able to adjust his shots on the run, I've just never seen anything like it.
"He's tough, he's mentally tough, he has the whole package.
"He's not looking at 14 (grand slams), he's looking beyond. Realistically he could well get to 17 or 18 majors when it's all said and done. He's a great credit to the sport."
Sampras said he could not see Federer's compatriot Wawrinka upsetting the odds to beat Nadal in Melbourne.
"It's a tough match-up and I'm not sure he has the firepower from the back of the court. He's got to deal with Rafa and his nerves, it's his first major final so he needs to settle down and work his way into the match.
"I think Rafa's just playing too well."
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- Stanislas Wawrinka