"I'm lucky that I have a very good metabolism. When I was 22, I ate whatever I wanted. I could have burgers four days a week and not gain one kilo. In the last couple of years, though, I've taken much better care of my diet.

"Breakfast is between 8:30 and 9—normally Special K with skim milk, fruit, and toast with ham or turkey. Then I go to practice. When I don't have a tournament, I'm on the court two to five hours a day, six days a week. Then I work out for another two hours, either at Reebok Sports Club La Finca in Madrid or at the hotel gym when traveling.

"Tennis is a sport that's very complete. You need to work every part of your body, so my routine changes—an hour running or on the bike or elliptical. For strength, I use everything: body weight, kettlebells, every machine in the gym. I'll do 500 crunches, 200 push-ups, 50 chin-ups . . .

"Throughout this, I'll eat a banana or a Herbalife bar, and I drink Endurox R4 carbohydrate and protein powders combined with water. I consume a lot of carbs and protein: salad with fish, or pasta or rice with chicken or steak. In Spain, we eat a lot of beans and legumes, too. I don't pay attention to calories, though. I eat what I feel like—but only the healthy stuff.

"Each practice has a different goal, but I always focus on my legs. Without strong arms, you cannot hit the ball, and the core is necessary to twist hard on your swing. But what's most important is to have good legs, so that you can get behind the ball. Once I worked my legs so hard that, after three days of practice in Vegas, I went to see Avatar with my coach, Gil Reyes, and my dad, and after the movie they were calling me from the end of the row, 'Let's go, let's go,' but my knees got stuck at 90 degrees—I was so sore I couldn't stand.

"I don't get distracted when I visit Vegas. I've tried beer and wine, but I've never enjoyed it, so I don't drink. When I party, I just have Coke, orange juice, or water. I'm kind of weird.

"Of course, sometimes I do indulge. Everybody has to know when is a good time to give your body and brain a little shock. When I'm on the West Coast of the United States, I'll say to myself, 'Today is the day I go to In-N-Out and have two burgers.'"