Tennis greats are asking how 19-times grand slam champion Roger Federer keeps on keeping on after 20 years on the professional circuit.
Mystified tennis greats want to know the secret to Roger Federer's extraordinary longevity after the ageless Swiss added to his legacy with another record-setting Australian Open run.
At 36 and 169 days, Federer is not only the oldest man in 41 years to reach the semi-finals in Melbourne, but also the only player in history to make the last four on 14 occasions.
And while his chief rivals were either too battered to contest the season's first major or have long since departed, Federer is showing no signs of slowing down.
Four-time champion Andre Agassi says he wouldn't be surprised to see Federer play until he's 40.
"I'm out of the business of predicting him any more because I've been wrong so many times," Agassi told AAP at Melbourne Park.
Two-time Davis Cup winner Wally Masur, now Tennis Australia's head of performance, doesn't want to predict Federer's next feat so much as gain a greater insight into how the 19-times grand slam champion has managed to retain his supreme level after 20 years on tour.
"I'd like to sit down with him and his fitness trainer and just forensically learn what was the plan," Masur said.
"Or how did you go about it? What was your training regime?
"That might be the great mystery because it mightn't be that sophisticated - or it might be really cleverly done.
"But I know for example with his fitness trainer, it's not 365 days a year. He has specific times where he has specific things he works on and then he takes responsibility.
"That would be a great read."
Adding to the legend - and intrigue behind his approach - is the fact that while Novak Djokovic is fastidious about his diet, the ultra-relaxed Swiss is often seen frequenting restaurants along the Yarra and mingling with fans on Melbourne's Southbank, totally at ease in tourist hotspots.
"This is maybe where Federer has got it stitched up. He can walk into an art gallery in the morning or the afternoon of a big match under lights at Flushing Meadows," Masur told AAP.
"He's got some sort of perspective that a lot of tennis players don't have.
"Other players put a racquet in their hand and go and hit balls again (and have little down time).
"Everyone does it but we're searching for that perfection, hitting it just right, but it doesn't exist.
"And I think that culture of having another hit, another warm-up, another set, another session, it's a bit like (Muhammad) Ali - maybe the damage wasn't done in the actual fight but all the damage was done in all the build-up and all the sparring he was doing.
"That's what tennis players do too. Just the hours of training."
Without knowing the ins and outs of Federer's meticulous work ethic and whether or not the Swiss marvel uses dieticians, nutritionists, chefs or if wife Mirka merely dons the apron and does the cooking to keep her husband in shape, Masur has another theory on why the father of four keeps on keeping on.
"Federer can win at 75 per cent," Masur said.
"Like a jogger running at 75 per cent, they can sustain it. But start going at 100 per cent and you're done.
"But look at Federer's great rivals, when you watch them play, I get sore just watching the way Novak and Rafa (Nadal) and (Andy) Murray, in particular, throw themselves around the court.
"Even Serena (Williams), it's such a physical game these days and the ball moves so quickly from A to B.
"I'm more staggered when they have a healthy year than when they don't.
"But Federer can win at 75 per cent. He's that good, and his serve is so good and he's got that many ways to win points.
"(Pete) Sampras was a bit the same. He could win at 75 per cent.
"But Rafa and Murray, even when they're playing in those first and second rounds, they have to get dirty; dig, fight, scrap, run and be Rafa and be Andy and be Novak.
"And that's where I think Roger has been able to see them off because he's just had this ability to be in a gear that didn't kill him but, when it really it mattered, he had higher gears to go to too.
"He rarely red-lines."