Editor’s note: This article is part of the Winning Style series, a collection of articles that explore the methods elite tennis players adopt to remain at the pinnacle of the sport.NEW YORK — In the minutes after Novak Djokovic lost as he has never lost before at the U.S. Open, the greatest male player of tennis’ modern era spoke about his attempts to conquer whatever it is that has been leaving him depleted and vomiting on the court on several occasions this season.“I’m trying, man, I’m trying,” he said in a hallway underneath Arthur Ashe Stadium. “Let’s see.”The opening night of the 2026 tournament was unlike any other in Djokovic’s career. He nearly gave up, he said, early in a five-set loss to Argentina’s Mariano Navone, the world No. 49.Instead, the 39-year-old played on. He swallowed pills to settle his stomach and manage his cramps. He cried on the court in the middle of a game — perhaps from pain; perhaps from the increasing futility of seeing through a match that had been gone for hours.Or, for the first time, Djokovic was seeing what his tennis mortality might look like at one of the few tournaments for which he is still playing the sport at all. The extraordinary longevity that has earned him 24 Grand Slam titles has also made him the only serious rival to Carlos Alcaraz and Jannik Sinner, the two players taking over the top of men’s tennis. That he is still on the court at all, let alone putting himself in the conversation for tennis’ biggest titles, is in many ways beyond measure.Messi and Djokovic: Two 39 year olds still owning their sportsLukas WeeseNothing about Djokovic’s dedication to routine, to recovery, to priming his body as much as he can for as long as he can and at the right times, has changed in the past two years. That’s how long he says a medical condition — that he has not disclosed — has been sapping his energy and making him even more susceptible to heat and humidity than he already was.It is the passing of time that is undoing him, those years and the near five hours he spent in Arthur Ashe Stadium against Navone, with close to 24,000 fans watching in an atmosphere that turned funereal in the fourth and fifth sets.For Djokovic, it had been funereal since the fourth game. The sight of his struggling left Carlos Alcaraz, who had a match that afternoon but caught some of Djokovic’s afterward, stunned.“A big surprise, to be honest,” Alcaraz said in a news conference. “Someone like Novak going out in the tournament, you know, in the first round, it’s surprising.”Ultimately, Djokovic is managing something that everyone experiences: aging. Great athletes experience it more keenly than most in their sunset years; ultimately, no one can topple it. He is also trying to figure out where tennis, and all the things he does to make that tennis the best it can possibly be, fits into his life.After years of playing 70 matches a year all over the world, the hours of training and recovery before and after and the endless travel, Djokovic also wants to be present as a father to his two children.He has played just 21 matches this season, 16 of them across the four Grand Slam events. Except in rare instances when players are at the peak of their powers, there is only one way to get into shape for tennis matches, which is to play tennis matches.When he was at the peak of his powers, Djokovic could time his surges like no one else, using early-round matches like the one against Navone to groove his groundstrokes and elasticate his limbs.Novak Djokovic’s elasticity and movement are fundamental to his mastery of tennis. (Clive Brunskill / Getty Images)Djokovic would be the first to acknowledge he is no longer at that peak. But around the time he first referenced the condition to which he attributes his most severe on-court energy drops, he also said, during a roundtable at the Cincinnati Open, that after making the semifinals of Wimbledon, his priority was to travel with his family in Europe.“It’s a normal kind of curve in the cycle of our journey as individual professional athletes. It’s what most of the athletes are doing, the younger ones today. That’s what I’ve done,” he said. “I don’t want to have that one-dimensional role anymore.”His training is far less structured now, he said, though he added that he remained active during his vacation.“I play tennis even when I’m resting; I play tennis with my wife, with my son, with my daughter,” he said. “I play with my brothers, and we have fun, you know, just trying to enjoy life.”That may be good for his soul, but will it allow him to maintain his status among the men’s tennis elite as his 40th birthday approaches? Regardless of the medical condition he is managing, age takes its toll on everyone.Dr. Robby Sikka, the medical director for the Professional Tennis Players Association — an organization Djokovic co-founded but has since parted ways with — said endurance doesn’t suddenly fall off a cliff when an athlete enters their late 30s. They just can’t do the same training, and they can’t recover from their training as efficiently.The engines of performance — max heart rate, muscle mass and fast-twitch fibers among them — wane. Replenishing the energy needed to fuel those engines takes longer, too.“The difference shows up when you ask the body to sprint, stop, change direction, recover — and do it again hundreds of times over several hours, and then potentially do it again two days later,” Sikka said. “Recovery between points, between sets and between matches becomes incrementally less efficient.”For Djokovic, who remade some of the fundamentals in elite tennis movement, most notably translating sliding from clay to hard courts, these infinitesimal declines accrete with every match he plays.Some of the conditions intrinsic to contemporary men’s tennis at the Grand Slams — five-set matches that can drag on toward midnight and then long after that — only exacerbate the inefficiencies that develop as time marches forward.“Tendons are stiffer, joints and muscles have absorbed decades of loading, and small injuries and degenerative changes accumulate. That can subtly change movement efficiency, so an older player may expend more energy accomplishing the same movement (as a younger one),” Sikka said.These changes can even help to explain the vomiting, though Sikka said he would not draw causation between it and aging. Everyone can struggle with nausea during intense exercise. The body reduces blood flow to the gastrointestinal system because it is needed by the muscles, the heart and the skin.Earlier in a player’s career, a five-set match might use up 85-90 percent of their energy reserves. In their late career, it might be closer to 100 percent, which means the body will start to drain blood from the digestive system sooner.“I wouldn’t say he’s vomiting because he’s older,” Sikka said of Djokovic. “I’d say recurrent nausea can be a visible sign of just how close to the limit the body is being pushed. And as the margin between sustainable workload and maximal workload narrows with age, it can potentially become easier to reach that point.”In his final years, Philip Roth, the writer, referred to aging as “a massacre.” And while Djokovic’s final years on the planet are likely a ways off, his final years as a professional tennis player are upon him. The tennis player who might have done more than any other to prime his body for the pursuit of greatness is learning the hard way that he is as susceptible to aging as any mere mortal.The Winning Style series is sponsored by Ralph Lauren.The Athletic maintains full editorial independence. Sponsors have no control over or input into the reporting or editing process and do not review stories before publication.