Roger Federer could play for another four years and add to his tally of 17 Grand Slam titles, according to former No. 1 Pete Sampras.
Speaking ahead of an exhibition match against Andre Agassi in London on March 3, Sampras said on a conference call that he is impressed by Federer's longevity.
"I'm amazed that he is up for more tennis," Sampras said.
"He has done everything in the game and he could walk away tomorrow feeling great about it. But he still wants to travel and compete and I'm in awe of it. He is a true lover of the sport."
Following a disappointing 2013 season marred by back problems, Federer started this year on a high.
Now working with new coach Stefan Edberg, he reached the final of his first tournament, at Brisbane, and then made the semifinals of the Australian Open for the 11th consecutive year. His run at Melbourne included wins over 2008 finalist Jo-Wilfried Tsonga and Wimbledon champion Andy Murray.
Sampras, who retired after winning his 14th Grand Slam title at the 2002 U S Open, said Federer should be comforted by a streak that finally ended with a defeat to Rafael Nadal.
"I thought his level was quite good," Sampras said. "When you're playing against Rafa and if you're not quite on your game then it can seem like you're struggling. Rafa is a rock.
But I do see Roger building from that, I think he's going to do well this year."
Asked if Federer could win another Grand Slam title, Sampras said the Swiss "can do it" if he plays his best tennis.
"That's why he's playing, I don't think he's playing for anything else but to win some more majors," said Sampras, adding that Federer's best chance would be on his favourite grass surface, at Wimbledon.
Sampras was 31 when he decided to hang up his racket for good, while the 32-year-old Federer has hinted he could keep playing until the Rio Olympics in 2016.
"As long as Roger is enjoying it and he's healthy, I can see him playing for another two, three, four more years," Sampras said.