London: 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."