Pete Sampras
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As the career of federer progresses, one has to wonder if he will one day be the consensus "best ever" on the men's side. As of now, I don't see how anyone could disagree that Borg is the best of the open era. That said, I would say that Federer and Sampras are in a tie for second place all-time. He was a truly great player.
Update: With Federer on the scene now, Pete is not even the second best anymore.