Aryna Sabalenka‘s team is filled with gems. But a standout favorite is her devoted physical trainer, Jason Stacy, who joined her entourage in July 2018. He is affectionately dubbed as “Grandpa.” But, Stacy’s legendary antics, like sporting tiger-striped temporary tattoos at the 2023 US Open and the 2025 Australian Open, mirror Sabalenka’s fierce spirit perfectly. And in terms of helping Sabalenka on the court, her records speak for themselves. Now, Stacy has crucial information for the fans.

On April 10, the fitness coach posted an enlightening story on Instagram, captioned, “Add this to your health wellness, performance plan!! (Men and Women!)” He shared a reel from ‘Mydiagnostics’ to raise awareness about the limitations of blood tests for hormone level checks. According to the podcast, blood tests can be misleading for one simple reason.

They noted, “When we go into our doctor’s office and they do a blood test, it can often be the moment that a woman becomes disempowered. Because we’re not often looking at the day of her cycle that they’re being tested at—it’s, as we talked beforehand, hormones pulse in. So, if the hormone was pulsing at that particular moment, it may show up as a certain level, so it’s such a weak test.” This is where the ‘Dutch Test’ comes in—a comprehensive hormone panel analyzing dried urine samples to measure sex hormones, adrenal hormones, and their metabolites.

The ‘Dutch Test’ could be a game-changer for athletes. The podcasters even tried it themselves and were amazed by the results. One claimed, “I ran one on myself every year, and I think the first time I ran a Dutch test, I was like, ‘Oh my god, I think we could end breast cancer.’ If every woman ran a Dutch test every single year.” For tennis players, both male and female, this test provides a vital hormonal profile that can optimize performance and well-being by assessing key hormones like testosterone and cortisol.

Shared by Sabalenka’s fitness coach, this advice could help a massive number of people. As part of the World No. 1’s team, Stacy brings a wealth of knowledge that has contributed to Sabalenka’s success, including 19 titles. Her raw power and aggressive baseline game leave opponents astonished, but how has Jason impacted her team?

Jason Stacy’s strategies as Aryna Sabalenka’s fitness coach

Aryna Sabalenka’s fitness coach has a unique approach called ‘Mindset & Movement Mentorship’ that emphasizes harmony between body and mind. “You can’t have mindset without movement or movement without mindset. How we move influences our mind, and how we think and feel influences how we move,” Stacy explained to Tennis Pal in 2019. For him, it’s all about crafting the perfect balance of elements tailored to each athlete’s needs. He added, “It’s about developing that character to pull the trigger when you need to.”

Stacy’s philosophy revolves around understanding the athlete deeply—both as a person and a player. He prioritizes learning their history, past coaching experiences, and personal aspirations. “We take a person-first approach. Secondly, we look at the athlete—who they are and how they move. Lastly, we look at the player—their position or style of play,” he shared. This holistic method has been pivotal in Sabalenka’s rise to World No. 1 and her stellar performances, including her recent Miami Open victory.

Though he didn’t grow up playing tennis, he quickly adapted to its unpredictable nature and thrived in its dynamic environment. His journey began with Dmitry Tursunov before transitioning to consulting roles for Davis and Fed Cup teams. Fate brought him back to tennis when Tursunov introduced him to Sabalenka. Reflecting on her growth, Stacy said, “She genuinely is driven and wants this so badly. Her mindset makes all the difference.”

Jason has been focused on refining Sabalenka’s movements to make her power more efficient. He’s working on ankle mobility, body awareness, and deceleration techniques—key for tennis players who constantly stop and start during matches. “Your body is only going to allow you to go as fast as it knows you can control,” he explained.

Now, Aryna Sabalenka heads into the clay season after a stellar victory at the Miami Open against Jessica Pegula. She enters as the World No. 1, ready to defend her ranking. Will Jason impart some secret knowledge and help her prosper on the red soil? Share your thoughts in the comments below!