A $690,000 watch is now part of Nadal's game

Tennis great Rafael Nadal is one of the most obsessive, ritual-oriented players on the tour. And part of that ritual now includes a $690,000 watch.

So when a little-known watchmaker asked if Nadal might consider wearing a watch during his matches, the answer was simple.

"I said no way," said Carlos Costa, Nadal's manager. "I know Rafa, and ... he's complicated."

In the year that followed, however, Nadal and the watchmaker Richard Mille worked together to create a superlight, hyper-engineered watch that Nadal could wear during his matches. In the process, they've also forged one of the most successful athlete-sponsor relationships in sports.

After selling out of the first Nadal watch, priced at $525,000 and launched in 2010, Mille and Nadal this year launched a second watch. It's called the RM027-01. The price: a whopping $690,000.

Mille doesn't disclose sales, but said "interest has been very strong" from its clients. As for Nadal, he not only wears the $690,000 watch during every match but says he can't imagine playing without it.

(Read more: The $500,000 Nadal watch is now sold out )

"I don't feel the watch," he said, strapping it on before practice at the U.S. Open on Tuesday. "It's very light and when I don't wear the watch, I feel like something is not working."

Of course, plenty of tennis stars have watch endorsements. Roger Federer has a deal with Rolex. Maria Sharapova helps sell TAG Heuer. But neither wear their watches on the court.

But for Nadal, the RM027-01 watch has been wrapped around his wrist for every match during what is shaping up to be one of his best years ever. If he wins the U.S. Open, he would become No. 1 in the world, with more than 50 wins this year and only three losses.

In 2010, Nadal's team and Mille spent more than a year developing the watch so that it would be light enough and strong enough for Nadal to wear on the court, Costa said. The result was the Tourbillon RM027, Mille's first "Nadal timepiece."

"It took about 12 to 15 months ... but Richard made it and it's unbelievable," Costa said.

The RM027 was a complex tourbillon-meaning the movements are designed to offset the effects of gravity. But it weighed only 20 grams, and is built from complex carbon composites and titanium.

But for the new watch, Mille wanted something even lighter and more tailored to Nadal's game. Nadal and his team spent a weekend at Richard Mille's home and talked over design colors and features.

Because Nadal's signature "whip" forehand has been clocked at over 4,000 revolutions per minute, they wanted a watch that was durable and could withstand huge force.

Mille created a radical new design, using a series of tiny cables, tensioners that "suspend" the movements in the watch casing. The result: the watch can withstand accelerations of up to 5,000 Gs of force. (To put this in perspective, anything over the force of 10 G is potentially lethal for humans.) And with the use of carbon nanotubes, aluminum lithium and titanium, it's only 19 grams.

(Read more: Highest-paid players in major American team sports )

Costa said that while Nadal loved the first watch, he likes playing with the second one even more.

"Between the first watch and the second watch, there's more technology," Costa said. "You know the weight, and the way he wears the watch ... it's just unbelievable."

And yes, Nadal said, that even when he's playing, he sometimes uses the watch to check the time.

-By CNBC's Robert Frank. Follow him on Twitter @robtfrank .

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