Last night’s US Open final showed brilliantly what makes today’s men’s game so exciting: Much as I love the serve-and-volley game, these long, kinetic, full-court exchanges of sharply angled groundstrokes make great viewing. How on earth, you ask yourself, are Rafa Nadal and Novak Djokovic hitting the ball so hard and keeping it in the court?

Part of the answer lies in the new generation of strings. As this video below from The Atlantic shows (and an accompanying story explains), 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 — while they’re grabbing the ball — to create more spin. (The key illustrations showing this comes at about 4:15 of the video. Some wonderful super slow-mo of Nadal and Federer generating spin follow directly, at about 5:00.)

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. (Some great slo-mo of this here. This is a huge jump over the spin rates of even his modern peers.

But all these players hit with spin almost no one could produce even a few years ago. What’s often missed, however — partly because it’s very hard to see unless you’re sitting live at one end of the court — is how much sideways spin these players often put on the ball. I was lucky enough to see Nadal play Jo-Wilfried Tsonga (possibly the tour’s best obscure player) this June at the Queens Club Tournament in London, a traditional grass-court run-up to Wimbledon. Like most people who see top-level tennis for the first time ever or in a long while (as was the case for me), I was stunned most immediately at how incredibly hard these men hit the ball and at how fast they are afoot. 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.

But perhaps the most amazing thing that jumped out at me was the sidespin these players put on many shots. 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. These players do this, I would guess, in about a quarter to half of their groundstrokes. If you watch them carefully as they hit, you can see how they do it — they sometimes swing through the ball sideways as well as up. As I say, it’s hard to see on vid, but you can see a hint of this in some of the forehands Nadal hits in the practice round shown below (there’s one he hits up the middle where you can see this particularly well): He’s hitting some strokes on the outside of the ball a bit, and the ball curves to his right as it arcs over the net. That motion will be exaggerated when the ball bounces:

Side spin is not new to the game. Big Bill Tilden, who dominated tennis 80 years ago,* used side spin a lot and wrote a lovely book about it, Match Play and the Spin of the Ball. And while few club players use sidespin on their groundstrokes, the stronger ones do, and most elite college level players make this a routine part of the game. A few years ago, when I was playing a decent level of club tennis, I had a teaching pro, Robert, who taught me how to use sidespin on forehand approach shots down the line: to add some clockwise sideways spin to the slice underspin, you had merely to pull the racket toward the center of the court as you sliced through the ball — a nice natural motion, even on the run. 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. This shot set up a lot of easy volleys and often ended the point, as my opponent mishit the ball or sent it wildly off court.

Robert also taught me a stronger lesson about sidespin. He had been trying to get me to come to net only behind strong approaches, and I made the mistake of following a weak one in. He had given me a short ball, and instead of hitting my fancy new sidespin approach to the open backhand corner, I hit a straight one up the middle. Robert set his feet, took a full forehand swing — horrifying racket speed — and hit a missile at my right hip. For a millisecond I thought I had a chance: Coming out of the split-step, I moved left to take the volley on the forehand side, and things briefly looked good. I could see the ball would dive to just below net height, but I thought I might get the racket on it and volley it toward his backhand corner.

Then the spin took effect. For Robert, undetected by my unschooled eye, had come not just up through but sharply across the ball, swinging outside in. When it got about 6 feet away it started bending sharply right (that is, to my left). It was no longer headed toward my racket, but toward my, uh, zipper. Fortunately the bend increased, and the ball found neither my racket nor my zipper but a rather tender area some three inches below my waistband and three inches left of midline, right where torso meets leg. I think it still hurts.

Robert found this wildly entertaining, and so did I. But I was also rattled, neurologically and phenomenologically, for when the ball bent so sharply, out of nowhere, the laws of physics seem to have broken down. It was vertiginous.

Of course, the laws of physics were very much in effect. And, amazingly, elite pro players don’t just routinely hit those shots but routinely return them.

How? How on earth do they compensate for this sort of movement? Doubtless through seeing the spin on the ball. But a bit of research from tennis as well as from baseball suggests they’re also picking up cues from the opponent’s hitting motion. In one baseball study I read a couple years ago — but somehow, dammit, cannot find right now — some clever researcher put elite college baseball hitters in a virtual batting cage. A big screen showed actual footage of a pitcher throwing a pitch and then the pitch’s trajectory toward the plate. But no hitter got to see all of that: hitters got to see either the pitcher’s motion up to the point of release, or the ball’s flight after release, and then swing at the (virtual) ball. They actually judged the pitch’s location better when seeing the only the pitcher’s motion than they did when seeing the actual trajectory of the ball. This doesn’t prove the ball’s flight told them little, since the virtual simulation of the ball’s flight might be (probably was) less realistic than the film of the pitcher’s motion. But it showed that they collected a lot of information from the pitcher’s motion.

So with tennis. Even good club players often know which side a serve is coming to before the ball is struck, and pros routinely anticipate which side a serve will come to. (Sometimes they don’t move at all; they’ve been fooled and were leaning the wrong way.) This is doubtless going on during ground strokes. If Robert hits 20 of those twisters at you, you start to pick up the sideways racket motion and accommodate.

And the pros are doing this all this time — and not just reaching the ball but responding in kind. (They also get a lot of information from sound — so much so that back in 1977, a nearly soundless string briefly warped the men’s game until it was banned. The string so infuriated Ilie Nastase in a match against Guillermo Vilas that he walked out of a final, forfeiting.) We club players sometimes think we’re playing the same game they are. We don’t even know.

**Tilden had big hands, a huge serve, and little patience with inferior opponents. Sometimes, legend has it, when he was serving someone badly overmatched, he would begin a service game by taking four balls in his left hand — and then keep them in his hand as he served four straight aces to win the game.