Novak Djokovic has had a flexibility coach since he was a child.
More than a decade into his career, no-one in the men's game has really been able to replicate his particular brand of rubbery athleticism.
His ability to run full pelt, slide into the splits and play powerful shots on both wings from positions most other players couldn't even reach is perhaps his biggest point of difference on an ATP tour full of sensational athletes standing at more than 1.9 metres.
So it's ironic that his rigid anti-vaccine position could be the thing that brings him unstuck in his quest to truly be considered the greatest male tennis champion of all time.
This is a man utterly dedicated to the sports science that has helped him win 20 majors, but his refusal to accept the epidemiological science that has so far vaccinated more than half the planet's adult population against COVID-19 may be the thing that stops him reaching 21.
We may never know what he truly believes, but in an exclusive interview with the BBC, the world number one spoke in depth about the fact that he had not received any COVID-19 vaccine and was willing to miss grand slam tournaments if the alternative was getting the jab.
In his conversation with BBC media editor Amol Rajan, Djokovic insisted he "completely disagree[d] with" anti-vaxxers and said he decided not to get vaccinated because he had "always supported the freedom to choose what you put into your body".
But in the same answer he implied he did not believe the vaccine would be healthy for him.
"I've always carefully reviewed, assessed everything that comes in [to my body] — supplements, food, the water I drink, sports drinks — anything that comes into my body as a fuel that I can benefit from. I take that very seriously.
"There's a process of precise decision-making. It has been like that for the last 20 years."
He also said he would "keep an open mind" about vaccination, without specifying how his mind could be changed. But with more than 10 billion doses of COVID-19 vaccine administered, it's hard to see how much more data could possibly be gathered.
Djokovic said point blank that if his vaccine stance stopped him from becoming the greatest tennis champion of all time, that was a price he was willing to pay.
And it's a good thing too, because that is almost certainly the RRP of his position.
Djokovic opened his grand slam account in 2008 — five years after Roger Federer and three years after Rafael Nadal.
By the time he won his second, at the 2011 Australian Open, Federer had 16 and Nadal had nine, but that was the year the Serb made his push for greatness.
He won three of the four majors, nabbed 10 singles titles and finished with an untouchable 70-6 win-loss record.
Federer and Nadal had been playing catch with the GOAT title for years but, like when your parents force you to invite the unpopular kid to your seventh birthday party, they now had to let Djokovic play. And he started hogging the ball.
Since 2011 he's the only male player to win three of the four majors in a year, doing so in 2015 and 2021, when he won slams on three different surfaces. Perhaps that had something to do with his willingness to remain active at the height of the coronavirus pandemic in 2020, including hosting his own super-spreader event.
He is also the only one of the "Big Three" to claim all four majors in the past decade and has more Masters 1000 titles (the biggest of the non-major tournaments) than anyone in history with 37 — one more than the Spaniard and nine more than Federer.
All that to say he could already legitimately claim to be the GOAT of men's tennis, and even if he slowed dramatically, there's every reason to think he would have the longevity to catch Margaret Court for the most major singles titles, man or woman.
And Court could all of a sudden be a pretty good comparison for Djokovic for another reason.
She is, by the numbers, the greatest winner the sport of tennis has known.
Her mark of 24 major singles titles is still untouched, and if that number is eclipsed, the 40 doubles titles that go along with it mean she will always have something over her modern chasers.
But in 2022, is that the first thing you think of when you hear the name "Margaret Court"?
Rightly or wrongly, her espousal of homophobic views is perhaps her greatest claim to infamy for anyone not au fait with tennis history. At the very least, publicly honouring her tennis achievements without controversy is difficult.
There's every chance now that this becomes the second line of his obituary (or maybe even tacked onto the end of the first in particularly cruel publications), especially if it stops him surpassing Nadal and Federer in the majors tally.
Maybe this will only affect Djokovic for a year or two (at some point hopefully COVID is not enough of a concern that vaccination is even a talking point). He's still officially the best tennis player in the world, but he is already 34. Ask Federer and Serena Williams about chasing history after your mid-30s. They've won five slams between them since turning 35 in 2015.
And even if he can get the numbers, public perception, as we've learned with Court, is a factor in assessing the relative value of an amorphous concept like "greatness", although it's possible he gave up on winning that battle a long time ago.
The good news for Djokovic is he's well versed in chasing Federer and Nadal.
The bad news is he's never passed them and, with this latest admission, maybe he never will.
