"I decided to be free," Orlando Cruz says with piercing clarity as he looks out across his home city of San Juan. The Puerto Rican fighter, who this month became the first boxer to declare publicly that he was gay, remains on the balcony of his condominium as a blue and humid sky darkens. Cruz ignores the drops of rain that glisten on his bare torso as he whistles to Bam-Bam, a cheerful sausage dog who jumps on to his lap. The 31-year-old then talks with increasing passion about his new-found liberty.

"They can call me maricón, or faggot," he says with a wry smile as he tickles Bam-Bam behind the ears, "and I don't care. Let them say it because they can't hurt me now. I am relaxed. I feel so happy. But to make this announcement to the whole world I had to be very strong."

Cruz flexes his tattooed arms while deflecting Bam-Bam's urge to lick his face. He might usually be besotted with his little dachshund but, now, Cruz is fiercely concentrated. On Friday night, in Kissimmee, Florida, he faces the most testing bout of his career, a WBO world featherweight title eliminator, but he needs first to explain the far harder struggle he has finally won over fear and prejudice.

"I have done well as a boxer," he continues before switching to Spanish so he might speak more evocatively. "I've only lost two of my 21 fights. I won those other fights but, all this time, I have been living with this thorn inside me. I wanted to take it out of me so I could have peace within myself."

Cruz glances down and it's easy to imagine him searching for an invisible wound. "You can't see it," he says of his hurt, "but it was here."

He taps his heart and recalls his bleakest moment. "People have died because of this," he says as he details the murderous aspects of homophobia on the lush and sweltering island he loves. "I am proud to be Puerto Rican, just like I am proud to be a gay man. But I was sad and angry a long time because there are two doors to death over this one issue. There is suicidal death – when a gay man cannot stand being unaccepted and takes his own life. And there is homophobic murder. In both these situations I want to be a force for change."

Cruz is such a warm and friendly man, and an unassuming fighter, that these words carry a jolting impact. He makes it sound as if he has personal experience of tragedy. "Si, si," he murmurs. "I lost one friend who was murdered by people who hated gay men. I was very angry then because homophobia ended his life in the most violent way. But I was also angry because, at the time, I was hiding this secret of mine."

The rain falls harder and Cruz stands up, almost reluctantly, as if not wishing to break the spell of his confession. "Let's go inside or we will look crazy – sitting in the rain." He gathers his boxing paraphernalia – scooping up the gloves and headguards, his trunks and socks – and ushers us inside the condo.

Cruz sits on his kitchen worktop. He cannot quite believe how his life has changed in the last few blurring days. "It's emotional for me, but I am also excited. I think I can be an example for people who are in the same position. I have received letters from people saying they have been afraid to come out of the closet because of what their families might think of them. Now, they say, I have given them courage."

He looks still more moved when asked who helped him find the bravery he needed to tell the world the truth about himself. "One person is very important to me. I've known him four-and-a-half years and he taught me to value myself. I won't say his name but he is like my angel. We discussed this whole situation and he told me about the positive impact it would have for me. In boxing it has been great and, in Puerto Rico, the reaction has been 90% good. So I owe him a lot."

Cruz might say that one word, "angel", in English, but he shakes his head when asked if he's thinking of his partner? "No. We separated but we still have this closeness. I am on my own now and he always tells me to focus on boxing. He's a good guy and he'll be at the fight in Kissimmee on Friday."

Kissimmee might sound a sweetly coy name for a gay fighter called Cruz to make his first appearance in the ring as a self-confessed homosexual. But boxing's brutal undertow cannot be forgotten. While Bam-Bam crunches his dog biscuits and laps noisily from his water bowl, Cruz licks his own dry lips. Boiling down to the 126lb featherweight limit, and only days from fighting Jorge Pazos, a durable and still ambitious Mexican, Cruz has to ration every morsel of food. And, despite his raging thirst, he'll soon step into the rustling sweatsuit that will help him shed more ounces during afternoon training.

Cruz poses with his dog Bam-Bam for a portrait in his apartment at Carolina Puerto Rico. Photograph: Herminio Rodriguez for the Guardian

Cruz's life has been turned inside out by his revelation and it seems strange that he should have invited such scrutiny so close to a fight of this magnitude for him. If he wins on Friday his hopes of fighting the world's best featherweight, the WBO world champion, Orlando Salido, will feel deliciously close to fruition. But a loss to Pazos would be disastrous. Was it difficult to come out so close to an important fight?

"No," Cruz says. "I wanted the whole world to know the truth about me. I have been a professional fighter for 12 years [having made his paid debut with a first round knockout win in December 2000] and I have been hiding this secret all that time. Now there is no secret. There is only the truth. Believe me: that means there is so much less pressure on me. It is so much better. I have been thinking about this moment for 11 years. All the time I was fighting and thinking when would be the best time to show my real self. It started in 2001 when I told my parents."

Cruz laughs as a way of easing his emotions. "You should have seen me," he says, remembering the moment he told his mother he was gay. "I was crying! She was crying! I am emotional and I am so close to my mother. She said: 'It doesn't matter. You are my son. I love you.' That made me cry some more."

Cruz pauses before addressing his father's reaction. He sighs, his breath leaving him in a muted hiss of resignation. "My dad is more difficult because of the macho thing. Now, it's better. He supports me but… there is always a 'but'..."

The fighter raises his eyes and there is no need for him to explain more. "My parents are separated. My dad lives in Miami but I'm glad he will be at the fight to support me. And my mother and I will fly together to Orlando. She was always more sympathetic – she's a special friend. And my sister and brother are the same. They have been great. They have all known for a long time."

His phone rings repeatedly but Cruz has been so engrossed that he waves dismissively at it. Eventually he picks it up on the caller's fourth try. "Oh," Cruz says in English, looking at his phone in surprise. "It's my trainer. The two o'clock call…"

On a public holiday in Puerto Rico, Cruz's usual gym has been shut for the day. Yet he had still set his alarm for 4.30 that morning. Thirty minutes later he had slipped out into San Juan's sultry blackness. What did he think about on his long and lonely 5am run? "I thought about the fight against Pazos. October 19 holds my future because if I win then the next fight is for the world title. So I go through the fight in my head, round by round, and I see myself knocking him out.

"Sometimes my team runs with me. But this morning it was just me. I had the space to think about everything. I moved to New Jersey two years ago because my manager wanted me to get disciplined. There are too many distractions in Puerto Rico. And when I was in New Jersey I started the psychological process of being able to come out."

Cruz seems briefly pensive as he charts the arduous journey he has taken to reach this point of release. "After a while the psychiatrists say: 'Are you ready?' I say: No, not yet.' A few months later they ask the same question. I shake my head. I was nervous a long time because it's a big step to be the first in history. Even six months ago I was worried how people would take it. I had to wait until I was physically and emotionally prepared.

"It was still a big surprise to a lot of people in boxing. But the response was good. Miguel Cotto [the great Puerto Rican light-middleweight who is the same age as Cruz and his former team-mate on the national amateur team] said some beautiful things in support of me. Miguel suspected I was gay but I could never discuss it with him. But I always knew Miguel would support me. I never doubted that."

Does Cruz believe that his coming out will help other gay boxers follow the same path? "I don't know. Probably in other sports it will happen. But boxing will still be difficult because it is so macho."

Cruz's face grows sombre and he nods when asked if he knows the tragic story of Emile Griffith and Benny Paret. "Of course," he says. Fifty years ago, in April 1962, at the weigh-in before their third bout in a bitterly ferocious series, Paret taunted Griffith as a maricón. Griffith beat up Paret so badly that the Cuban welterweight was reduced to a punching bag in the 12th round – absorbing 29 unanswered blows. Paret fell into a coma and died 10 days later. Griffith was haunted for decades afterwards.

"Griffith was gay," Cruz says, "but he could not do what I did. It was only years later he could admit to being bisexual. I understand."

Cruz listens intently while I read a quote from Griffith – who said these words before he succumbed to dementia: "I kill a man and most people forgive me. However, I love a man and many say this makes me an evil person."

He sinks back into his chair, a strange expression flitting across his face. "It shows the hypocrisy of the world," he murmurs in Spanish. "He probably wanted to say those words 50 years ago but he was not living in the moment we are now. He was not as lucky as me."

Cruz carries a sense of boxing history inside him and cites Muhammad Ali as his favourite fighter. He covers his face in embarrassment when I suggest that, in his own humble way, he has made the kind of history that usually belongs to fighters as monumental as Ali. Cruz has not risked jail, like Ali did in refusing to serve in the US Army in Vietnam, but he has broken the last great taboo in boxing.

"Thank you," he says before lightening the moment with a quip. "Even women here in Puerto Rico were surprised. They used to say to me: 'Oh, you are beautiful!' Now they say: 'Oh my God! You are gay! I'm sorry!' But they accept it. They are still nice and warm."

When did Cruz realise he was gay? "Before the 2000 Olympic Games in Sydney I tried to deny it to myself. I dated girls as a straight man. I had sex with girls. It was only after I came back from the Olympics that something changed inside me and I took another path. But, still, I didn't want to accept the truth about myself. It's been a long, painful journey."

At the sound of his doorbell Cruz jumps up. "You're going to meet my father-in-law," he says. Jim Pagán is a veteran of the ring, having trained Puerto Rican fighters like Eric Morel and Cruz for years, and he arrives at the condo with a weathered face and a quiet gravitas. Cruz tells me how Pagán, who speaks little English, has trained him since he was seven years old. "Twenty-four years," Cruz exclaims, as he reflects on their bond.

Cruz jumps rope during his training prior to his fight with Orlando Salido. Photograph: Herminio Rodriguez for the Guardian

Another more emotive bond ties the two men together. "I went out with Jim's daughter for five years," Cruz says. "Her name is Daisy-Karen and she has supported me. Just like Jim."

With Cruz acting as translator I ask the trainer how he feels now that his daughter's former boyfriend has come out as a gay man. "We have great respect for each other," Pagán says in soft but gravelly Spanish. "I have always known Orlando is a very good person."

Cruz laughs. "Not always," he says, switching back into English. "He once told me to fuck off and leave his gym. I had no discipline as a kid. But I always came back to him. He's my second father."

Walking in tow with Pagán's two young sons – one who hopes to become a professional fighter while the other dreams of playing baseball for a living – Cruz leads us to a gym at the far end of the complex. It is neat and clean and without any of the grit and stink of Pagán's boxing gym in downtown San Juan.

Inside, Cruz skips with a rope and then smacks his fists into Pagán's raised pads. They make eerie shadows when silhouetted against the fading afternoon light; but the old tattooed beat of their pad-work calls up a shared and enduring love of boxing. Cruz is now just a fighter preparing for a dangerous battle.

During a brief break, I ask if he feels nervous. "Not yet," he says. "The worst is two hours before the fight. Oh my God! Then there are big nerves. I go very quiet. But as soon as the knock comes on the locker-room door I am fine. And on Friday I will be ready."

Once the fight is over, and he has hopefully secured his crack at Salido's world title, Cruz will party a little in Kissimmee. "And then," he grins, "I go to Disneyland in Orlando with my mom. She loves it."

Cruz might get hurt or pushed to the edge of his ability against Pazos. Yet he insists that, after the greater struggle he has just won in real life, he will prevail in the ring. "Pazos is a tough, typical Mexican fighter. We respect each other. When they asked him about me he says he doesn't care about my sexual preference. He knows I am a good fighter and that's his main concern. I am the same towards him. I keep my private and professional life separate but for one thing..."

Cruz looks up, his eyes shining in his sweat-streaked face. "If I am inside or outside the ring I just want to be me. And, now, I'm happy I can do it. I can be true to myself."