"Again and again one player would hit a ball so hard and at such an effective angle and so deep into the court that it was obvious the other player couldn’t reach it. Then the other player would reach it–and hit another irretrievable shot that was retrieved and returned. The athleticism of these players is almost inconceivable... Their shots don’t merely dive hard onto the court. They often curve sideways as they dive and then explode from the bounce with sideways motion... When the ball bounced, it would skip not just up (flatly, because of the underspin) but jerk sideways toward the sideline — and away from the player who was, if you did this right and hit the ball deep enough, on the run and trying to hit the ball on the short hop."
The video below is an excellent illustration of this stunning combination of power and skill.
As David Dobbs writes, technology has played a significant role in this transformation in the nature of tennis groundstrokes.
"The co-poly strings in use today — which spread through the pro game only over the last decade or so — generate more spin than ever. They do so because they’re more slippery than prior string designs. Because the strings easily slide across one another, they can slip back and then snap back to position — all while they’re grabbing the ball — to create more spin... Thus Nadal, Djokovic, and their peers can hit the ball harder than ever and still generate enough topspin to bring it down into the court. Nadal in particular generates enormous topspin — an average of 3200 rpm, and as high as 4000... This is a huge jump over the spin rates of even his modern peers."
See this excellent Times video which illustrates how Nadal generates his topspin. Incredibly, Nadal's average topspin of 3200 rpm is much greater than Federer's 2700 rpm and double Samparas's 1700 rpm.