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 Wednesday that he is impressed by Federer's longevity. ''I'm amazed that he is up for more tennis,'' Sampras said. He is a true lover of the sport.'' Following a disappointing 2013 season marred by back problems, Federer started this year on a high. More...