In a concrete bunker in a corner of a favela, the sweat is dripping off the walls. Instructions are being counted out with a frenetic, relentless rhythm.
"Um, dois, tres, quatro, cinco, seis, sete, oito, nove, dez alterar!"
The boxers and mixed martial arts fighters change and repeat, change and repeat. They are going to "lugares escuros" ("dark places"), one will later say. It is their sweat that is slicking the walls and concrete floors.
Nobody at the Academia Nobre Arte wants to be the first to break. Not the short or tall, the boxers or MMA specialists, the men or women, the gay or straight.
The lone New Zealander does not want to break. He is shadow boxing, skipping, hitting the heavy bag. He doesn't understand the instructions, so takes visual cues and never seems more than a micro-second behind.
He also moves differently to the rest. He is taller, more naturally athletic perhaps, certainly more graceful. An older head, a coach, tries to get him using more shoulder, less arm, when he punches. The light-heavyweight listens politely but ignores him.
"Homosexual," says a local onlooker, pointing to the coach. It is not a term of disparagement but a matter of fact and a signal that in this city of religion, where Christ the Redeemer looks down in permanence on its citizens, this academy of noble arts is a broad church.
On this day, David Nyika has joined the congregation.
Nyika is hoping this won't be his last trip to Rio de Janeiro. The 20-year-old missed a chance to qualify for the Olympics when he missed a medal at the AIBA world championships in Doha this year, beaten in the first round by powerful Uzbek Rustam Tulaganov.
It was a setback, but only a minor one. Nyika thought he won - two judges scored the three-round bout 29-28, the other 30-27 - but learned a valuable lesson in the process.
"You can't leave those decisions to the judges, especially when you come from New Zealand, because nobody knows who you are."
If there's a note of defeatism in that appraisal, that's the limitations of the printed word playing tricks on you. Nyika knows exactly what he wants from his amateur career and how to get there. He is that rare mix of committed athlete and detached observer.
He devours the art and skill of boxing, but can take or leave, most likely leave, the business behind the professional arm of the sport.
"It's not something I need to do. If I'm going to turn pro, it's going to be because I've become the best amateur boxer I can be and I'm ready to sell out," he says with a self-awareness missing in most starry-eyed young sportsmen.
"I'm going to try to do everything for my nation before I go and sell myself as a brand. As an amateur boxer, you have the opportunity to build yourself before you jump in the lion's den.
Depending on how good you are, there will be people who want to throw money at you and throw you into the cage. I have enough freedom to take my boxing as far as I can. I'm not a materialistic person; I don't need a lot of money."
The amateur-professional dichotomy is a curious one. If you cream off the top 10 per cent of pro boxers, those left behind are journeymen, fall-guys and rogues.
Amateurs learn to box; pros learn about a food-chain that has the promoters at the top and the guys who bleed at the bottom.
"Everybody at the world champs had to qualify to get there. There were no bums. If I turned professional, I could fight 20 people I'd never fight as an amateur because it's a business and your record is marketable.
"It doesn't interest me to fight somebody that has never had a fight before, who is already hurt and could get hurt even more for a pay-cheque. There are a lot of people out there selling themselves to get beat up because it is quick and easy money."
In amateurs, Nyika faces a kindred spirit he says is difficult to explain ... before explaining it perfectly.
"You form a bond with your opponent that you can't really form in any other sport. It's really primitive in that sense," he says. "You don't run a race and turn to the person next to you and go, 'Whoa, that was pretty cool, pretty intense.' You're so connected in those nine minutes, it's unreal."
The professional fighting industry has been romanticised by some of the best writers in the business, but the truth is that it's a malodorous environment where perspiration competes with the stench of desperation.
An environment where the combatants have back stories that change only in the detail. Where the theme remains the same: kids trying to fight their way out of poverty.
Waikato via Uganda
Nyika doesn't have that kind of story. One of four children, he comes from a perfectly respectable postcode in a perfectly respectable town in a lucky country.
Which is not to say his background is boring. Nyika's father, Simon, is half-Ugandan, half-Welsh; his mother Susie is half-English.
"I've got a good blend," he jokes.
His paternal grandparents left Uganda for New Zealand, he thinks, during the Idi Amin reign of terror, but he makes a mental note to talk more with his grandfather about his life.
He was Hamilton born and largely bred, though his parents took the family to England for three years when David was seven.
"I was home-schooled there for about six months by my [maternal] granddad - that was pretty weird. I didn't like that so much, as I didn't have the chance to make any friends and then when I did make it to school, I didn't know what to do."
The family returned to the Waikato and Nyika slotted back in with his same friends and took up the same sports. His father was a boxing fan, so that was in the background, but it was athletics that lit his competitive fire.
He wanted to be an Olympic long-distance runner and was convinced his east African heritage could give him an advantage, until joint pain forced him to quit.
"I always wanted to be something a bit special. I come from a middle-class background and that's something that has always driven me; to be something that's not middle-class, not ordinary.
I've never been one to want to settle down with a nine-to-five job. I made it a priority to do something a little unusual."
An escape from middle-class sensibilities is not something that drives many boxers but Nyika is not many boxers. The anger does not come naturally.
"That's an interesting one. I don't get angry in the ring. There are times when you have to find another gear than laidback. I've always been pretty cheeky. My brother [Josh, 24] and I always used to fight and I'd have to do something fairly sneaky to get one up on him. If I can have fun in the ring, am enjoying myself and relaxed, then I'm probably as dangerous as I'll ever be."
Ahead of Glasgow, however, Nyika found something to get angry about and used it to exploit a hitherto untapped wellspring of emotion.
"My parents split up six months before the Commonwealth Games and I used that ... Your parents are meant to be the best people in the world. They raise you from a baby to an adult and it's a bit of a shock when something like that happens.
"There are a lot of people and a lot of boxers in particular who grow up in tough backgrounds and you hear all about it, but that was something pretty big for me, so I used it to spur me on. We're still good with each other, mum and dad are still good, it's just something I could use ahead of Glasgow."
Use it he did. Nyika, who only made it to Glasgow via a circuitous route that included tournaments in Poland and Albania, won gold after five wins in nine days. It was a spectacular arrival announcement.
"The road to Glasgow was pretty bumpy. It was tougher than it should have been because I was primed to peak a number of times and the fights fell through. It was rubbish."
The hoop-jumping has at least prepared him for the trials of getting a spot on the Olympic team.
Know when to quit
A man with too much time on his hands once said: "Thinking will not overcome fear, but action will."
It could be boxing's mantra, but it would also be a foolish one. In this day and age, when we have seen a thousand punch-drunk pugilists come and go, thinking is about the best weapon you have. A helping of fear will probably do you more harm than good, but ultimately it is skill that will prevent damage.
"When you go into the ring, you need to have a balance between being confident and cautious. The fact we work so hard on my defence, everything happens instinctively. I don't go into the ring thinking about where my opponent's punches are coming from and how I'm going to dodge them - it just happens.
"There's only been a couple of times when I've been a little bit worried, and one of those was going into my Commonwealth final. I was hyped up, but I remember warming up and I suddenly realised how deep I was into the tournament. It was the first time I'd stopped to think about it. My body was sore, my shoulders were sore, my back and arms were sore. I was thinking, 'whoa, I'm pretty beat up'.
"I walked out and saw the crowd and I was like, 'this was crazy'. The fact a lot of good boxers had fought this guy from Mauritius [Kennedy St Pierre] and really struggled, had been hurt by him, I thought this guy must be pretty good."
Nyika backed his defence and the result was a convincing win.
"It is a product of my training. I've been taught to move a certain way and I've translated that to suit the language of my body. You have to move and relax.
If you move and you're tight, it's going to count for nothing because as soon as you start thinking about where you're going and how you're going to get there, you're going to get hit."
Nyika's Commonwealth gold could be described as the base camp of his amateur ambitions. He was in Rio with Sky Next, who sponsor athletes they recognise as having the drive and ambition to keep their name in lights for the long term.
It helps, too, that Nyika is movie star good looking. While in Rio, he cuts several clips for the network and is a natural in front of the camera, but he's got things to do inside the ring before he thinks about the next phase of his life.
There'll be buckets more sweat and a lot of "dark places" to go before the gloves are hung up. But he won't leave it too late. He's too smart for that.
"Boxers are the first to know when to quit and the last to admit it," former WBA featherweight champion Barry McGuigan once observed. That's a trap Nyika will avoid.
Dylan Cleaver was in Rio de Janeiro courtesy of Sky Next. Sky is the broadcast home of the 2016 Rio Olympics on both Sky Sport and Prime TV.
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