"Until you do it you can't comment on how difficult it is. It's tough. It's ugly," says Andy Murray.
The British number one isn't talking about facing Roger Federer's forehand or Rafael Nadal's two-handed backhand - he's talking about Bikram yoga.
Since December, on the suggestion of his new fitness coaches Jez Green and Matt Little, Murray has been preparing for the Grand Slams by practising Bikram.
And when Murray defeated Federer, the world number one, in Dubai in March the Scot credited the effects of the yoga technique.
But you can forget the clichéd image of yoga practised by hippies and housewives for relaxation; invented by Bikram Choudhury more than 30 years ago, this yoga is intense.
Each 90-minute class is held in a room heated to 40C and sweat rolls into the eyes as practitioners grapple with the sequence of 26 repeated postures.
Yoga for the masses: Murray has been attending public classes
Murray's not the only elite sportsperson to incorporate the hardcore yoga into their training regime.
Fellow tennis players John McEnroe and Serena Williams are fans, Chelsea footballer Wayne Bridge and Fulham's Moritz Volz and Simon Elliot have visited the west London studio, while the whole of the New York Giants NFL team are devotees.
"A lot of athletes are turning to Bikram yoga because it undoes the wear and tear," Michele Pernetta, who introduced Bikram yoga to the UK, told BBC Sport.
"Most sports have long-term effects but yoga is the science of putting the body back the way it's supposed to be."
To prove how beneficial Bikram can be to all sports practitioners, Pernetta put a high-level martial artist, runner, boxer, rugby player and golfer through a week of classes to see how much damage sport had done to their bodies, and how Bikram could help mend them.
After seven days, all the athletes showed improvement with noticeable loosening of their joints and improved flexibility.
Your mind is going, 'I'm not good enough, this is really awful', but you have to deal with it
The UK's Bikram expert Michele Pernetta
Murray, the world number 11, has already enthused about the immediate results he's seen as a relatively new student.
"It's helped a lot with my fitness and mental strength," said the 21-year-old Scot. "If I keep doing it then in about six months I think there'll be a huge improvement in my flexibility."
The rewards Murray is already reaping start off on a physical level.
Tennis players can suffer repetitive strain injuries to the wrist, elbow and knee and tend to have imbalanced bodies with a dominant left or right side.
Bikram yoga stretches and strengthens the muscles and the heated room means there is less chance of an injury - good news for any athlete competing week-in, week-out at the elite level.
"The sequence of postures and the heat is unparalleled in bringing oxygenated blood to 100% of the body," explains Pernetta, a former martial artist.
"Bikram works your internal organs, your cardio-vascular system and detoxifies the body whilst continuously nudging your skeletal system back into alignment.
Pernetta helps a runner release his hamstrings in the floor bow pose
"A high-level athlete like Andy will have incredible intensity levels, and so Bikram will either help him release that or help him focus. It will also allow him to open up his joints, his hips and back."
As well as the physical gains, yoga also helps control the breath, improve focus and calm the mind.
That could also be of some use to Murray, who has complained about becoming "angry and frustrated" on court this season.
"Bikram yoga is really hard," adds Pernetta. "You go to places that are not only physically uncomfortable but that are emotionally uncomfortable too.
"Your mind is going, 'I'm not good enough, this is really awful', but you have to deal with it. Long-term, yoga is really focusing and teaches a form of honesty."
The Lawn Tennis Association has been so impressed by the effects of Bikram that it is not just sending Murray to practice; many up-and-coming players are now regularly attending classes too.
LTA osteopath Sophie Scott told the BBC: "Bikram yoga has been extremely beneficial to our players.
. . and what he is aiming for, a perfect Bikram floor bow
"It provides a warm environment to engage in a routine of exercises that help flexibility, mobility, control and coordination - all key elements for a tennis player."
Olga Allon has been teaching Murray and his fellow British players in public classes at her west London studio, and is now helping elite athletes from other sports train for this summer's Olympics.
Allon described Murray as "pretty good" for a beginner and believes all the elements of Bikram will help improve his game over time.
"He can take all this onto the tennis courts with him," said Allon. "Keeping a relaxed breath, using flexibility to jump for balls and keeping a poker face during matches.
"Andy will find Bikram tough but doing something so intense and maintaining the focus and attention should help him concentrate in any situation."
But Bikram is not just for elite sports stars like Andy Murray, it can be practised by anyone.
Bikram himself has succeeded in demystifying the technique and turning it into the fastest growing form of yoga, with the most male practitioners, and a multi-million dollar industry in the United States.
Pernetta, who says 40% of her clients are male, set up her first studio eight years ago and is now overseeing 10 studios in the UK, with more on the way, while Bikram now expects to have 5,000 studios worldwide under his wing.
"The body will put up with a lot while it is young," warns Pernetta. "But the problems we encounter when we're older, are laid down now.
"Yoga can cure that."