"It's a huge goal, it's a lot of majors, a lot of work."
When asked whether Nadal needed to pad his resume to cement his place among the sport's all-time greats, Sampras said, "Quite honestly I don't think he needs to."
"He's won all the majors. He's won the Olympics. He's dominated his main rival, in Roger," Sampras said, referring to the Spaniard's 14-7 head-to-head edge over the Swiss master.
"I don't think his goal is 16, or 17 or 18, he's just going to try to improve as a tennis player and if it happens, great.
"He could do it, it's a lot of work ahead and he works so hard in every match he plays. But he's a beast."
That does not mean Sampras is dismissing the 29-year-old Federer's chances of adding to his own total.
"He's a strong favorite for every major he plays. He had two match points against (Novak) Djokovic," Sampras noted about Federer's five-set loss to the Serbian in the U.S. Open semi-finals.
"He could very well have been in the final. He's playing fine, I don't see any big decline. Other guys are playing better. The next two or three years is a challenging time.
"But Roger is up to the task and can win more majors."
Sampras would not be drawn into a debate about the best of all time.
India alone responsible for CWG mess: Hooper"Everyone wants to name the one guy. The way I look at all sports, especially tennis, is that each generation has their own guy.
"In the '60s it was Laver, you had Ivan (Lendl) and John (McEnroe) in the '80s, myself and Andre in the '90s. It's hard to answer. It's just that each decade has their guy," said Sampras, later throwing the likes of Bjorn Borg and Don Budge into the mix.
"But Rafa is definitely up there. You got to definitely put him in the top three or four just on what he's been able to do and it's not over yet. He's in the middle of his career so he can do a lot more great things."