It’s early afternoon in Orlando, the hottest time of day on a characteristically sultry Wednesday in Florida, a state famous for its perennially warm, wet, shirt-soaking conditions, which just so happen to be pretty much perfect for playing tennis.

Or so I’m told.

I’m standing—sweating, squinting, panting—at the opposite end of a court from 21-year-old Ulises Blanch, one of the many elite athletes who train here at the United States Tennis Association’s National Campus. I’m here to learn about the upper limits of the serve, the most nuanced stroke in tennis and one of Blanch’s specialties. I tell him I’m ready. He toes the baseline, lobs the ball into the air, and sends it bolting past me. “One hundred thirty-one,” says the speed-tracking system. From across the court I see Blanch grin. Sadistically, I think. It’s his seventh serve, and his seventh ace.

Blanch possesses a tremendous serve, yet it remains far from the most powerful. It’s been clocked at 138 miles per hour, which 30 years ago would have put him in the running for the biggest hitter in all of tennis. But serve speeds at the professional level have been climbing for decades. The 1990s saw the first official serves in the 140s. By the early 2000s, they were in the 150s. The fastest serve ever recorded came in 2012, when Australian Sam Groth was measured walloping a ball at 163.7 mph. But the Association of Tennis Professionals doesn’t recognize Groth’s serve, because he delivered it at a challenger event, where, according to an ATP spokesperson, serve-speed guns don’t adhere to the same standards as the ones used in tournament play. The fastest serve recognized by the ATP was delivered at 2016’s Davis Cup by American John Isner, at a speed of 157 mph.

“There are three big factors in optimizing for speed,” says physiologist Mark Kovacs, an expert in serve mechanics. “Technique, technology, and height.” The sport’s latest generation of athletes, he says, have pushed the limits of all three.

A former tennis pro himself, Kovacs works with some of the best players on earth to help them wring as much power as possible from their serves. The technical elements of a stellar stroke, he says, are well understood. You need strength, obviously, but flexibility is equally important—particularly in the upper body.

During a serve, the majority of a player’s power originates in their legs, but conveying that power through the body and into the racquet requires stockpiling additional energy in the hips, lumbar, and shoulders, by rotating all three elements in sequence as the ball rises into the air. Tennis types call that rotation coiling. A big serve requires a limber, practiced player—someone strong and loose enough to twist their torso taught like a rubber band and uncoil themselves a fraction of a second later, with timing so precise that it not only translates the energy from their legs but augments it.

Hard hitters like Blanch excel at storing and releasing energy throughout their bodies in this way. But they’re also working with more power, in general. While a typical amateur might produce between 700 and 900 newtons of ground force with their legs, Kovacs says the most propulsive pros can generate upwards of 1,500.

There was a time when tapping into that kind of power on the court was risky. With older, wooden racquets, which dominated the game of tennis for much of the 20th century, serving too hard significantly increased your odds of overshooting, sending the ball out of bounds.

But in the mid-1970s, manufacturers began blending carbon fiber and resin to produce racquets with bigger heads. The surface area of your typical racquet increased from 70 square inches to well over 100. That expanded the racquet’s sweet spot, which made the game easier for amateurs. But the pros didn’t need a bigger sweet spot. For them, larger, modern racquets have had a different effect: The ability to put more topspin on the ball. Thanks to a phenomenon known as the Magnus effect, a ball with more topspin dives toward the court at a sharper angle once it has cleared the net. Putting more spin on their serves allowed players to lower their risk of launching the ball beyond the bounds of the service box when they hit harder. The result: more powerful play and faster serves—especially for bigger, stronger players.

Today, the hardest hitters in tennis are all tall. Blame physics. “If you’re tall, it means you've got longer levers, so you can store energy over a greater amount of time, and that allows you to summate your forces over a greater distance,” Kovacs says. The spindliest of players have an even greater advantage: “Anyone under about 6-foot-7 is still hitting up on their serve.” But above that height, a player can reach high enough to actually strike the ball down at their opponent. “If you're hitting up on the serve, you're fighting gravity. If you're hitting down, gravity’s helping you.”

Not surprisingly, the five fastest serves ever recorded were pommeled by players 6-foot-4 or taller. John Isner, who delivered the fastest serve recognized by the ATP, is 6-foot-10. Ivo Karlović, who held the record before Isner, is 6-foot-11. But while serve speeds rose steadily throughout the ’80s, ’90s, and early 2000s, improvements in the past decade have grown less consistent and more incremental. Kovacs says that one reason for the plateau is that Isner and Karlovic have been playing for a long time, and up-and-coming players have yet to contend with their records.

Whether any of them ever will remains to be seen. Not that there aren’t prospects: American Reilly Opelka’s serve has been clocked at 145 mph, and he’s only 21. His secret? “Good coaching, good mechanics,” he says. “Also, being seven-foot helps.”

But Opelka says that if the current speed record ever does fall, conditions might play as great a roll as height, technique, and equipment. The fastest serves happen at high elevations and in hot, muggy climates. (Air density decreases as altitude, temperature, and humidity increase, which reduces drag.) “Honestly, that's why John Isner's been so successful in Atlanta,” Opelka says. Because his serve, which is already huge, “is even more enhanced there.”

And yet players don’t seem to place as much stock in speed as you might think. “It’s overvalued,” Opelka says. “Nobody at the tour level pays attention to it.” Instead, he says, players gossip about one another’s tendencies. “The talk in the locker room before a match, between friends, will be like, ‘This guy loves his slice serve—last time I played him, every big point, or under pressure, he was hitting his slider.’"

Blanch agrees: Speed isn’t everything. Back in Orlando, we’re chatting at the net when he tells me he only recently learned how fast he was capable of serving; his coaches used to hide it from him, so he wouldn’t fixate on it. “I’m actually working more on placement these days,” he says with a shrug.

Easy for him to say. Even in Orlando’s hot, humid, speed-friendly conditions, the fastest serve I could manage was 77 mph. I blame my height.

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