In the true spirit of the U.S. Open, where as much careful consideration is given to what to eat as to which match to watch, top-seeded Novak Djokovic's new book fits right in.

It is called "Serve to Win: The 14-Day Gluten-Free Plan for Physical and Mental Excellence." In it, the six-time major-tournament champion attributes his tennis success to a change in his diet and dispenses advice on nutrition.

The book has been described as equal parts autobiography, recipe collection, self-help guide and philosophical manual. Always a demon for fitness, Djokovic writes in "Serve to Win" that he remained "heavy, slow and tired" until, three years ago, he turned his eating habits away from pizza, pasta, bread, heavy meat dishes and candy-bar snacks.

He was cheered, he said, by the almost immediate feedback from spectators during the early-August tournament in Montreal. Naturally, the book is for sale on the National Tennis Center grounds, where anything is for sale in this vast, eclectic marketplace.

Especially food.

"Of course, I want to make sure that it's not for everybody," Djokovic said of the gluten route.

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Indeed, among his professional colleagues, accustomed to traveling with nutritionists as well as physiotherapists and coaches and other handlers, there hardly is diet conformity.

"I am a cheagan," Venus Williams said, playing on the vegan label. "I cheat a lot. You see a picture of me eating the wrong thing, that's why I have already confessed."

She likes "green drinks and smoothies."

Rafael Nadal, Djokovic's rival at the top of the men's rankings, cautioned that while "now seems like the gluten-free diet is great, after three years or four years we will find another thing that will be great, too. Then the gluten-free will not work anymore."

Judging himself always to have been a "healthy eater," 17-time major-tournament champion Roger Federer nevertheless declined to "take it to the next step," as Djokovic has.

"I'm happy I can eat a bit of everything today," Federer said. "Nothing crazy or major, really."

The pros for years have emphasized good dietary sense to facilitate fitness. Andy Roddick, late in his career as the top-ranked American male, cut out fast food and late-night meals to drop 15 pounds and improve mobility. As far back as the 1980s, Ivan Lendl supplemented exhaustive training with the then-fashionable Robert Haas "Eat to Win" diet. (Which prompted John McEnroe to kid that he personally favored "The Haagen Dazs diet.")

"I don't do the gluten-free deal," said Coco Vandeweghe, the 21-year-old American still looking for the edges that get her deeper into Grand Slam events -- she lost her first-round Open match Wednesday. "But I work with a fitness company that devises a meal-plan for me. You can have a burger. But maybe a turkey burger. It's not quite as good, but . . ."

To Nadal, "at the end of the day, all the small things can help if you don't get crazy. I always had the theory that the most important thing is to be happy, enjoy what you are doing, and be fresh mentally. I don't say it's negative. But if I have the gluten-free or perfect diet, [it's] supposed to be a big sacrifice for you and that makes you [unhappy] the rest of the day, better don't do it."