Rafael Nadal of Spain starts his serve during his men's...

Rafael Nadal of Spain starts his serve during his men's singles second-round match against Rogerio Dutra Silva of Brazil in the 2013 U.S. Open, held at the USTA Billie Jean King National Tennis Center in Flushing, Queens. (Aug. 29, 2013) Credit: Getty Images

What Rafael Nadal can do with a forehand groundstroke can make your top spin. Listen to this:

"It takes about a second for Rafa's ball to get to strings," said John Yandell, who has used high-speed cameras to study tennis ball rotation for years. "From .85 to 1.2 seconds. In that time, the ball turns over 50 or 60 times."

That is from 3,000 to 3,600 revolutions per minute. "It's hard to believe that could actually be true," Yandell said. "But it is."

Yandell, who publishes the online website tennisplayer.net, first filmed this sort of thing at the 1997 U.S. Open. And what he has seen evolve in the sport, with the introduction of polyester strings and the presence of exceptionally fit, athletic players such as Nadal and Novak Djokovic, is "the way spin has changed the contact height" so the game "now is played in the air."

"These guys are exploding up off the court and rotating their bodies up and twice as far," Yandell said. "What the spin has done is turned tennis into super-physical games of Ping-Pong on a big court."

This is as good an explanation as any why Nadal (against France's Richard Gasquet) and Djokovic (vs. Switzerland's Stanislas Wawrinka) are clear favorites in Saturday's Open men's semifinals. Beyond career head-to-head records -- Gasquet is 0-10 against Nadal and Wawrinka 2-12 against Djokovic -- is what Nadal and Djokovic can do with the ball.

Especially Nadal. His mean topspin forehand singles him out. And Djokovic, perhaps the most physically and strategically flexible man on tour, "at this point is the only player -- well, maybe Andy Murray, to some extent -- with the capacity," Yandell said, "to trade shots with Nadal.''

The two former Open champs -- Nadal in 2010 and Djokovic in 2011 -- deal in the currency of hard work, as both are known for their extensive training regimens. Still, for the rest of the tour players, there is no getting away from that unique Nadal topspin forehand.

It surges up from his toes, "more of an arc, heavier but just as fast" as the typical forehand, Yandell said.

"There is no such thing as pure topspin; all have some sidespin as well. And the other thing that makes Nadal so tough, if you look at his big bolo swing he uses about half the time, my hypothesis is he's increasing the sidespin.''

Even Roger Federer has been tormented by Nadal's topspin forehand in losing 21 of their 31 matches.

Against the Nadal forehand, opponents are forced to strike the ball at "shoulder level or higher at times," Yandell said, "and they may be a foot off the ground when they hit it. So controlling the contact height is the key to having a chance against Nadal. You either have to move way back or do what Djokovic has done successfully: Move in and take the ball on the rise, before the full effect of the spin destroys your contact point."

And it likely is about to leave another foe reeling.

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