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

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