As the pros hit the courts for the start of the U.S. Open, tennis beginners and aces alike are burning calories and practicing their lobs, drop shots and backhands with cardio tennis.The non-stop group fitness class is based on tennis moves, so everyone can get a workout while improving their strokes.And there is no need to retrieve that errant ball. With cardio tennis, the balls just keep coming.”Cardio tennis is a great workout using your tennis skills,” said fitness expert Denise Austin. “It’s almost like interval training, or a boot camp class on the tennis court and it encourages people to play tennis.”Cardio tennis is played on a tennis court and led by a tennis pro. But unlike a traditional tennis lesson, the emphasis is on movement rather than technique.”They blast a boom box on the tennis court and you just start doing these fun drills to music,” Austin said. “You’ve got running, sprints, squats, side-to-side shuffles and great tummy strengtheners with the different strokes. You’re moving the whole time.”Austin, who has been in the fitness industry over 30 years, says for the beginner, it’s a better workout than a traditional tennis game.”If you’re not a really good tennis player it’s hard to get in a good workout (in a traditional game of tennis) because you’re always missing the ball,” said Austin, a member of the President’s Council on Physical Fitness and Sports.Michele Krause, of the Tennis Industry Association, said cardio tennis was launched in 2005, in part to change the perception that tennis could not provide a good workout.”With doubles, you’re lucky to hit two balls in a game, especially at the recreational level,” she explained. “With cardio tennis you’ll hit 1,000 balls in an hour, and your heart rate is elevated the whole time.”Krause said men burn an average of 500 to 1000 calories an hour in a cardio tennis class; women burn 300 to 500.”It’s still about the sport, because it’s all about hitting a tennis ball, but it’s not about instruction,” she said. “Even though a certified tennis pro leads the class, he’s not expounding on how to hit the correct forehand or backhand.”Instead the format mimics most group fitness classes.”There’s a warm-up, cardio drills and games, then cool down. It’s very fast-paced” Krause said.Classes can even employ heart rate monitors, and props like agility ladders and various types of balls.”It’s quite different from traditional tennis,” Krause said, “but you’re playing games that simulate traditional tennis.”She said cardio tennis will improve your tennis game because you’re constantly repeating movements and strokes.”Tennis pros around the world have done forms of this since the beginnings of tennis. It’s how the elite train. We didn’t create something brand new. We formatted it, we packaged it, we marketed it,” she said.Today cardio tennis is in over 30 countries.Austin, who is based in Washington, D.C., married into a family of tennis players. Her husband, Jeff, was a professional player and her sister-in-law is Tracy Austin, winner of the 1979 U.S. Open. She said when the Austins get together, they play cardio tennis. All skill levels together.”I ‘m an average player and I play with good players, but even the good players are missing the shots because you’re running at full speed,” she said.But it’s cardio tennis, so no one’s keeping score.