By Julia Andersen

(CNN) — As seven-time grand slam singles champion Venus Williams said upon her comeback to the sport late last month: “Do you know how hard it is to play tennis? … You’re running the whole time, lifting weights and just, like, dying. And then you repeat it the next day.”

Fellow American Frances Tiafoe voiced a similar belief last year: “It’s the hardest sport in the world,” he told Sky Sports.

So, what is it that makes tennis so challenging? Are singles tennis players the fittest of all athletes in any sport?

Sports scientist Dr. Mark Kovacs believes so, heartily agreeing with Tiafoe. The former tennis player has dedicated years to researching fitness, training and recovery methods across sports and has worked with athletes from the NBA, MLB, US women’s national soccer team (USWNT) and WTA and ATP Tours.

Speaking to CNN Sports, Kovacs admits he’s slightly biased due to his affinity for the game he grew up playing, but also has decades of research experience to back his belief that tennis players are among the fittest – and toughest – athletes in the world.

Technical and physical attributes

It’s hard to think of a sport that requires such an extreme spectrum of skills as tennis.

Maximum oxygen uptake (VO2 max), maximum heart rate, resting heart rate and heart rate variability to analyze recovery are usually the focal points of aerobic fitness monitoring.

Most athletes know their VO2 max and you might hear the measurement thrown around quite a bit when the Tour de France comes around or you’re watching the Olympic Games.

VO2 max is measured as milliliters of oxygen per kilogram of body weight per minute of exercise. Put simply, the best aerobic athletes – like professional cyclists and marathon runners – are likely above 70ml O2/kg/min mark.

“Low 60s is usually what we see, which is comparable to most soccer players… so they’re at the top tier,” Kovacs tells CNN.

“(But) Tennis players will easily beat a professional cyclist or marathon runner in pretty much everything outside of VO2 and lactate, sometimes.”

Tennis is not a marathon; each match is a series of short sprints in every direction over multiple hours. Tennis at the elite level, though, additionally requires cat-like agility and explosiveness in the blink of an eye to react to an opponent’s latest shot.

“They’re (tennis players) not going to be at the top of any one physical attribute. They’re not the strongest. They’re not going to bench press like an NFL lineman would. They’re not going to squat like some of the top powerlifters do.

“But they do need significant leg strength because they need to produce, you know, 140 mile an hour serves, which requires power. So, their power training is a focus. They have to have muscular endurance at some of the highest levels.”

Despite their contrasting styles, the top two men’s tennis players – Carlos Alcaraz and Jannik Sinner – are prime examples.

“Alcaraz is a lot more speed, power, explosiveness. He’s got that piece that is his strength,” says Kovacs. “Whereas Sinner, his ability to be stable in any position on the court and decelerate effectively is something that makes him so good.”

Back in June, Alcaraz mounted a historic comeback after being two sets down and facing three championship points against Sinner in one of the greatest men’s French Open finals ever. Research like Kovacs’ has shown the negative impact fatigue has on hitting accuracy – and this was five-and-a-half hours of the best physicality and mental stamina on display.

“That endurance piece on the men’s side, especially because of the longer length of matches in the grand slams, is a requirement,” Kovacs explains.

“You can have the most speed, the most power, the most strength. You can have the best mind. But if your aerobic capacity, if your work capacity is not really, really high, you’re not going to be able to last in five-set matches.

“If you don’t have the endurance, the work capacity, you’re never going to be at the top of the game.”

‘Tough as nails’

It was also world No. 1 and then No. 2 matching up in the women’s final at Roland Garros. Eventual winner Coco Gauff also rallied following a shaky start to beat Aryna Sabalenka in three sets.

“Sabalenka is a phenomenal athlete. Strong, powerful, moves fantastic. And, you know, is able to hit the ball as hard as anyone,” Kovacs says.

“Coco Gauff, who I’ve spent quite a bit of time with over the years, is one of the best overall athletes in any sport. You could put her on a soccer pitch, and she would handle that just fine. You could put her in track and field, and she would crush it.

“She’s an unbelievable overall athlete, across speed, agility and her endurance is phenomenal.”

Kovacs says he’s often asked who the best players to work with and coach are and his answer is always the same.

“Women’s tennis players because they’re tough as nails. They handle high, high volume,” he says. “Their ability to suffer in a good way is something that makes them great.”

The suffering players endure in a tennis match, on the WTA and ATP Tours, is exacerbated by the incessant tournament calendar and, particularly at the grand slams, extreme temperatures.

“It’s the hardest sport in the world to play professionally because tennis players travel over 100,000 miles a year to compete in their sport. That’s more than double Major League Baseball and the NBA,” says Kovacs.

“Tennis players, because of the major international travel, have significantly more challenges with jet lag, with recovery from jet lag, with adapting to environmental changes.”

The sports scientist adds, “And the season’s 11 months long, it’s the longest season of any sport in the world.”

Mental game

The international nature of elite tennis leaves little time for practice and recuperation, taking a toll on even the best.

It’s not just the extreme physical toll of hours on the court and grind of the gym day-in and day-out. Tennis also requires a massive amount of mental fortitude.

Kovacs points to Venus Williams’ willingness to step away from the game as a key reason she’s been able to reinvigorate herself well into her 40s.

“Whereas other players would play the 11-month season and play every week or close to every week, they (Venus and Serena) would take three months, six months off.” Kovacs explains. “That obviously helps with longevity because one, your body’s not being beat up for as much time. But two, mentally, you’re able to find those breaks, those sabbaticals from the sport where you can focus your interest on other things and then come back and be refreshed and still enjoy the grind.”

British No. 1 Jack Draper has spoken openly about feeling “quite an anxious human being,” particularly after bouts of nausea and vomiting affected him in last year’s US Open semifinal against Sinner and in this year’s edition in his first round win over Federico Agustín Gómez.

“Tennis especially, or just any sport, is hugely mental and physical. I try my best all the time to keep on evolving, to keep on learning, and it’s definitely something I’ve had to just work through my whole entire life,” the British star said last year.

“I think I’ve got quite a strong mentality and I use up a lot of mental energy a lot of the time because I want it so badly.

“That doesn’t necessarily help a lot of the times, especially in these five-set matches and that sort of anxiety and those feelings can build up.”

Twenty-year old Alexandra Eala similarly discussed how building experience and resilience were key to clinching her first round win at the US Open on Sunday. After coming back from a break down and securing her first ever grand slam singles win on her fifth match point, Eala said: “I think that mental strength and that focus are the keys to coming out victorious.”

Tennis is a brutal sport of always thinking at least a step ahead of your opponent for hours at a time, all to ensure you can do it all over again the next day (or if you’re lucky enough to get a day off, two days later).

Alongside boxing and MMA, also tactical one-on-one sports, singles tennis puts a large amount of mental stress on players.

“Having to mentally develop these strategies against different players and different styles, and also the need to think through it. That concept is quite unique,” Kovacs says.

“If you look at the top of the rankings, you see the best athletes in general. You know, they’re also the best mentally as well. That’s what makes them the best.”

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