NEW YORK—Talk about an exclusive club. At the entrance a burly security guard electronically scans photo IDs for approval, and you better be a playa. Once inside, anyone with a body-fat percentage of more than three or four percent is scorned. Don't even think about making idle chitchat, as everyone's focused on dropping acid—the lactic variety. There’s no music or video monitors, and it wouldn’t be smart to get used to the place, as this ultimate before-, during- and, after-hours hotspot only rocks for two weeks a year.

Welcome to the U.S. Open fitness center, located on the third floor of the same gigantic building that houses Arthur Ashe Stadium. Every player, from top seed to lowest qualifier, spends time here. The front of the place looks like a tennis bag convention, as winners and losers from every match make their way here, usually directly from the court. They’ve also likely been here before their matches. To this crowd, maintaining their perfectly fit bodies takes more than eating right and getting lots of cardio.  
“For professional tennis players, fitness is the one thing you can control,” says Jay Gooding, national coach at the USTA Training Center East, who works with Melanie Oudin, and Christina McHale, among others. “You can’t fully control your forehand or how well your opponent plays, but you can control your fitness.”  
He’s preaching to the choir when it comes to this crowd. During tournament time, the place is crowded from 9 a.m. to sometimes well after midnight. Players are almost always accompanied by their trainers, who proceed to take command once inside. The place is huge, with stationary bikes, treadmills and all kinds of state-of-the-art fitness equipment. Ironically, the pros spend most of their time on the floor, stretching and being stretched. Believe it—ballet dancers got nothin’ on these folks. And they don’t do 10 minutes and split. Well, they do split, especially Kim Clijsters, but the sessions usually last half an hour or so.  
And not just after matches. When TV and stadium viewers see players arrive on court and hit with each other for a few minutes before starting a match, the players already had thorough warm-ups beforehand. At tournaments like the U.S. Open, it can get frustrating and tiring for the pros. On Day 2, Christina McHale and Kiki Bertens were scheduled for the fourth match on the Grandstand, not to begin before 3:30 p.m. McHale had a warm-up around 3 p.m., while 10th seeded Juan Monaco was playing Guillermo Garcia-Lopez. Monaco quickly took the first two sets, but then lost the third. So McHale had a second warm-up. Garcia won the fourth set as well, necessitating warm-up number three for McHale (and Bertens too, likely). The fifth set went on and on into a tiebreaker, and McHale unbelievably needed a fourth warm-up. Finally, the ladies took the court almost five hours after they might have. Bertens won in three sets. Guess where they went after the match?  
“There comes a point sometimes when fitness overtakes skill level,” says Gooding. “Look at the last Australian Open, where (Novak) Djokovic beat (Rafael) Nadal. It took six hours. At that point it wasn’t tennis anymore. It was survival of the fittest.”  

—Michael Catarevas