It's the finals of the 2005 U.S. Open, Federer serving to Andre Agassi early in the fourth set. There's a medium-long exchange of groundstrokes, one with the distinctive butterfly shape of today's power-baseline game, Federer and Agassi yanking each other from side to side, each trying to set up the baseline winner... until suddenly Agassi hits a hard heavy cross-court backhand that pulls Federer way out wide to his ad (=left) side, and Federer gets to it but slices the stretch backhand short, a couple feet past the service line, which of course is the sort of thing Agassi dines out on, and as Federer's scrambling to reverse and get back to center, Agassi's moving in to take the short ball on the rise, and he smacks it hard right back into the same ad corner, trying to wrong-foot Federer, which in fact he does--Federer's still near the corner but running toward the centerline, and the ball's heading to a point behind him now, where he just was, and there's no time to turn his body around, and Agassi's following the shot in to the net at an angle from the backhand side... and what Federer now does is somehow instantly reverse thrust and sort of skip backward three or four steps, impossibly fast, to hit a forehand out of his backhand corner, all his weight moving backward, and the forehand is a topspin screamer down the line past Agassi at net, who lunges for it but the ball's past him, and it flies straight down the sideline and lands exactly in the deuce corner of Agassi's side, a winner--Federer's still dancing backward as it lands. And there's that familiar little second of shocked silence from the New York crowd before it erupts, and John McEnroe with his color man's headset on TV says (mostly to himself, it sounds like), "How do you hit a winner from that position?" And he's right: given Agassi's position and world-class quickness, Federer had to send that ball down a two-inch pipe of space in order to pass him, which he did, moving backwards, with no setup time and none of his weight behind the shot. It was impossible. It was like something out of The Matrix. I don't know what-all sounds were involved, but my spouse says she hurried in and there was popcorn all over the couch and I was down on one knee and my eyeballs looked like novelty-shop eyeballs.
David Foster Wallace on Roger Federer: Federer as Religious Experience, from The New York Times, August 20, 2006.