Tennis is not just beautiful, it is hell. Ask Rafael Nadal, who still does not trust his wounded knees after seven months hobbling in the shadows.

Listen to John Isner, whose body has given up on him on the eve of the Australian Open. Maria Sharapova's collarbone aches. Heather Watson is nursing an elbow strain. Jo-Wilfried Tsonga's hamstring nags him, as does Victoria Azarenka's toe, Tommy Haas's groin, Kei Nishikori's knee, Janko Tipsarevic's wrist and Richard Gasquet's mind.

The only given about the first major of the year is that both champions will suffer in the extreme, more obviously in the men's tournament. The winner could spend more than 25 hours on court over the next fortnight, much of it in draining heat, all of it stretching his resolve and muscles to the limit.

A year ago, Novak Djokovic took 10 minutes short of five hours to beat Andy Murray in the semi-final and, after the briefest of rests, a further five hours and 53 minutes to win the final against Nadal. Within six months, the accumulated stress from that match and thousands of others had so wrecked Nadal's crumbling knees that he was being hustled out of Wimbledon in the second round by Lukas Rosol, who was ranked 100 in the world at the time.

It was the last time we saw the Spaniard on court and, after his recent decision to delay his comeback until next month in Acapulco, there are growing concerns that, at 26, he might be entering the final phase of his career.

As for those glad of his absence in Melbourne and still in full working order, they have arrived like marines fresh from boot camp, their drill sergeants still barking out instructions. Without backup, they could not compete – which is why players outside the elite often struggle to keep up.

Djokovic relies heavily on Gebhard Phil-Gritsch and Miljan Amanovic to maintain his fitness; even the languid Roger Federer bends to the regime, the longest-serving member of his team being the conditioner Pierre Paganini, who has been with him on and off for more than 15 years; and Murray has Jez Green.

Green, who works alongside Matt Little, looks at Murray as not just a friend and fine player, but a maturing tennis machine the team have to ensure reaches maximum efficiency at the right time.

"Andy is moving towards his physiological peak from 25 to 28," Green says. "He achieved a 200 beat-per-minute maximum heart rate. His maximum minute ventilation is indicative of a highly trained athlete and has increased since last year's test. His oxygen pulse, which is an indication of how much blood and oxygen he can consume for every beat of his heart, is still extremely high.

"The fact that Andy has now put more emphasis on strength and power in his training and still has such a high VO2 max (peak oxygen uptake) is unique. Not many people can improve so much on strength, put on a few pounds of predominantly muscle and still maintain such a high VO2.

"It's usually very difficult to put on muscle and improve endurance, as they counteract each other in training – that is, muscles needs extra protein calories to get stronger and obviously endurance training actually burns those calories. Andy has improved endurance statistics and put on three pounds of lean muscle. In our lifting sessions, we did twice a week from September to November, 2011. We did four times a week in the same time in 2012, resulting in another three pounds of muscle. We just felt he needed a little more strength to keep improving his ability to create force to help his speed, shot power and also injury resistance.

"I've been training Andy for five years, and I still find it hard to believe all the things that he can do. He is naturally fast, but he is also so strong: on his best day he can do 27 pull ups, and push 500 pounds on the leg-press. He could probably run 53 to 55 seconds for 400 metres if he trained for it. He is a big, powerful guy, whereas Novak has a wiry strength; his flexibility is extraordinary."

Murray cracked that famed Djokovic strength over four hours and 56 minutes for his breakthrough win at Flushing Meadows, although the world No1 was reluctant to concede later that it was the Scot's tactics of forcing him to retrieve repeatedly across the baseline in the first two sets that laid the foundations for his win. Djokovic, famously conscious of the slightest twinge and once not so indestructible, did not want to give up that hard-won aura of physical invincibility.

"It's a tough time to be a tennis player," Green says. "What you're seeing in this generation is a group of guys with incredible genetics: you can try to play like Roger Federer, you can imitate his footwork, but he is just born with something that sets him apart. The work you do as a fitness trainer is about trying to maximise that."

Brad Gilbert, who once trained Murray, says: "Your heart rate has to go up to the sky, come down in a second and then go right back up. And somehow you have to psychologically convince yourself to suffer like that for six hours. You've got to be like an iron man in a triathlon."

Green points out, "Andy's training is very specific. We've never done a bench press, for instance, because tennis is all about back strength. It's a pulling sport, not a pushing sport, in that you use your big muscle groups to pull the racket back and turn your body into a spring. Then you just unwind and drive with your legs."

Nevertheless, talent and instinct remain crucial. "Andy has lazy speed," Green says, "by which I mean that he doesn't look as if he's moving that fast, but it's actually deceptive. He's been clocked at moving at 10 metres per second over very short intervals, maybe even as short as a single step, which is as fast as Usain Bolt. I'm not saying that he is that fast over 100 metres, but he has great acceleration when he is chasing down a drop shot.

"Even more valuable than his flat speed is the ability to stop and turn so quickly. He's putting three times his bodyweight through his legs in that moment, so they have to be seriously strong. But above all he is fast with his eyes: he picks up the cues so quickly and he knows where the ball is going that much faster than almost anyone else. All the guys in the top 10 have that ability to some extent. These guys are so strong they can compensate if they have a problem: it's almost like they switch over to other muscle groups like a computer running a different program."

Murray showed that when he was reduced to serving at 60 miles an hour in the first set to get past Jarrko Neiminen in the second round at Roland Garros last year with a back temporarily racked by seized-up muscles. Virginia Wade's intemperate "drama queen" jibe that followed could not have been further wide of the mark.

Ivan Lendl, recognised as the player who took physical training to another level in the 1980s, also has had an input in Murray's preparation. One of the first things he did when he took up with Murray a year ago was to get him to cut down on the arduous work he did on his serve. Murray would often serve for an hour or more on the practice court, but Lendl asked him, "Why? You don't want your arm to fall off. Save it for the match."

The notion of players as physical beasts might be at odds with the historical aesthetics of tennis, but Nadal's extended rehab and the Australian Open sicklist provide stark evidence that, in this rat-on-a-treadmill era, it could all go horribly wrong in the snap of a ligament, or the fading of the will.

Over the next two weeks, we will again bear witness to their commitment not just to beautiful tennis, but to exceptional, unprecedented physical sacrifice.