WTA World No.1 Serena Williams came in at No.5 on Sports Illustrated’s Fittest 50, a list of athletes across all sports disciplines that an expert panel named as the fittest in the world.

The 21-time Grand Slam champion’s overall fitness was evaluated and her unmatched strength, power, agility and quickness stood out to SI’s experts, as well as her longevity: at 34 years old Williams now owns 70 career titles and doesn’t look ready to slow down any time soon.

To come up with the Fittest 50, the editors of SI worked with a panel of trainers, strength and conditioning coaches, performance experts, pro athlete coaches and more. Athletes were assessed based on characteristics that define fitness and conditioning: strength, speed, endurance, agility, flexibility and skill in their respective sports.

No.22-ranked Sloane Stephens also joined Williams on the list at No.38, and it’s no surprise why with three WTA titles under her belt this year already. The American’s fitness routines are well-documented on her social media: she typically does a morning workout in the gym followed by a couple of hours of work on the tennis courts.

SI’s experts highlighted her gym routines, which include battle ropes, Bosu balls and pull-ups.

Visit Sports Illustrated’s Fittest 50 for a complete list of the world’s fittest athletes in all of sports.