He told BBC Sport: "I think he's incredible. I saw him play live for the first time on Friday night and the movement he has is incredible. He's able to adjust his shots on the run. I've never seen anything like it.
"Realistically, he could very well get 17, 18 majors when it's all said and done. He's a credit to the sport. I'm a fan."
Only Roger Federer, with 17 Grand Slam titles to his name, has won more Majors than Sampras.