It's happening. Albeit eight years too late.
I actually don't know who I like in a match-up of Sampras and Federer both in their primes. These theoretical tennis matches are always dependent on lots of things--for instance, are you talking about a player at his absolute single-moment peak, or at some rough approximation of how good they were during the meat of their career? In other words, if you're talking about a player at their very best moment, I think you have to consider McEnroe, who was downright scary in 1981. But if you consider the totality of his game, he's a top 10 (or 12) all-time player and isn't in league with Laver, Borg, et al.
But Sampras at his general best level against the Federer we've seen for the last couple years? I just don't know. Federer is such an amazing ball-striker, Sampras was probably the best pure athlete ever to play the game. Both could switch on their dominance and both had the ability to play a game basically independent of the guy on the other side of the net.
At this point in time, I give the smallest edge to Pistol Pete II, because he was so mentally tough. But I'm willing to be persuaded otherwise if Federer is able to assert himself over the coming wave of great young players who will be reaching maturity in the next 24 months or so.
(Andy Murray and Richard Gasquet, in particular, who look like monsters.)
3 hours ago