Tennis is a serious mental challenge—but above all, you must stay in shape.

Do tennis players face one less inevitability than the average human?

Age has never seemed as negotiable as it does right now. Cosmetic products and enhancements, nutritional revelations, medical advancements—if 50 is the new 40, then what does it mean to be 50, 60, 70 or older on a tennis court? How long can we continue to play a recognizable version of our own games, and what are the keys to tennis longevity?

Here’s the bottom line for anybody who wants to play well into middle age and beyond: be fit, or else. Conditioning preordains much about any player’s game, but it becomes even more paramount with age.

“Every one of us is fit,” says 68-year-old Bob Litwin, former No. 1 in the 55-and-older category and author of Live the Best Story of Your Life: A World Champion’s Guide to Lasting Change, of his top-flight peers. “People who stay fit play very similarly to how they played at 35 or 40,” he says.

“Keeping the weight off is huge,” says Joe Perez, 60, Co-Director of Tennis at CityView Racquet Club in New York City. Perez’s doctor told him to drop ten pounds with this admonishment: “In a match you take 1,500 steps; multiply that by ten pounds.”

How one maintains fitness is an increasingly personal decision with age, usually dictated by what sorts of injuries an individual is prone to. Perez, who’s had three knee operations, prefers non-impact biking, light, upper-body free-weight training and core work on non-tennis-playing days. Litwin, on the other hand, prefers to stay fit by playing, focusing his off-court efforts on stretching.

Stretching prior to a match also becomes more essential in staving off injuries. David Slater, formerly of Old Oaks Country Club in Purchase, NY, recommends a light warm up—jogging around the perimeter of the court or a few minutes of mini tennis—before attempting to stretch. “Stretching cold doesn’t do you any good,” says Slater. “It’s like stretching a cold, elastic band.”

Sure, anybody with a booming serve or knockout groundstrokes will always have a puncher’s chance, but staying in top shape allows for the speed, flexibility and power everybody requires on the court, at any age.