Oscar Pistorius strides through the sun-dappled car park in his designer suit. He has expensive sunglasses perched on the end of his nose, and shoes of the softest leather on his feet. He looks like a lawyer or TV executive. Certainly nothing like his alter ego, Blade Runner, dubbed the fastest man on no legs, and one of the most incredible athletes the world has ever known. But when the 25-year-old double amputee changes his “day legs” for his “racing legs” – those metal, J-shaped contraptions that earned him his sobriquet – he’s ready to take on the world.

At the Olympic Games he ran in the 400m and 4 x 400m relay, becoming the first double amputee to do so. “I loved the experience of running in the Olympics, but I’m all about the Paralympics now,” he says. “I’m very, very proud to be a Paralympian.”

Pistorius has broken conventions, ignited debates and made it impossible to ignore Paralympic sport. Above all, he’s compelling, and hugely successful. Not just on the track, but off it as well.

He lives in Pretoria, in a Mediterranean-style house in a gated community. A raft of sponsorship deals and commercial tie-ins with the likes of Oakley, BT, Nike and Thierry Mugler mean that he’s one of the highest-earning athletes in the world, and certainly the most commercially successful Paralympian ever. He's charming, good-looking and was recently voted the best-dressed man in South Africa by GQ magazine.

So what is the man behind the big build-up really like? I caught up with Pistorius before the London 2012 Games and found a man utterly devoted to his sport and to winning.

“I just want to run as fast as I can,” he says. “I want to be treated like any other athlete and I want to be able to challenge myself and perform at the highest level. As soon as you push boundaries, you meet with opposition, but I’m not doing anything wrong. I want to make the most of my God-given talent. That’s all.”

What is most striking when you speak to him is his winning mindset. “If there’s something I can do to be faster, I just do it, it’s simple.” The most obvious sign of that pursuit of perfection is the 19 kilos (3 stone) he shed in the three years running up to the Games. But look harder and you’ll see an extraordinary attention to detail.

After every training session and particularly after every race, he writes down minuscule observations about his performance. “I write down what the weather was like and how training went beforehand. I write down how I got there, including whether the train was late or the traffic was bad. I write down what I ate and how I was feeling and details of any injuries, how much rain there was that day. Everything. I do it so that when it comes to a race in the future, if I’m not feeling great, or haven’t eaten well, I can look back and know that the last time I ate badly, I still managed to win the race. Every race is won or lost in the head, so you have to get the contents of your head right. Writing things down helps you to control your thoughts.

“Being a perfectionist is everything. If you skimp, you lose. With me, it’s a mindset, and I’ve always had it. I joke that if I toast bread it has to be perfectly toasted. I suppose it’s a bit OCD in many ways, but that’s the way I am. I just don’t like short cuts. If I do something, it has to be done properly.”

Pistorius was born in Sandton, Johannesburg, without either fibula (the slender bone that runs from the knee to the ankle). His parents were strongly advised to have both his legs amputated before he got used to life with them. “They were told that it would be easier for me to adjust if my legs were amputated before I could walk,” he says. As a result, he became a double amputee before his first birthday, and he doesn’t remember a time when he did have legs. But this never stopped him from playing sport.

“I won a trophy for Greco-Roman wrestling when I was six,” he says. “I remember taking up boxing when I was around nine. It felt like nothing was off limits.”

When Pistorius was 13, he went to the highly respected Pretoria Boys High School, where he competed in triathlons, water polo, cricket and tennis, always playing against able-bodied children. He said he never felt different from the other pupils, and he certainly never felt inferior.

“Mum had a no-nonsense attitude to me,” he says. “She loved me and she also had high expectations of me.” He tells a story about her saying to his older brother, Carl: “Put your shoes on.” Then saying to Oscar: “And you put your legs on.” Her normalisation of his disability and her encouragement were important in his formative years.

But when he was 15, his mother died suddenly from a drug reaction following a hysterectomy. It was a devastating time. He says his mum is still with him, every day and in everything he does. He has the date of her death tattooed on his arm. He says she taught him to believe in himself.

After his mother’s death, Pistorius threw himself into school life. He loved rugby, but when he shattered his knee in a match in 2003, he was told to give it up and take up track running to recuperate. He did just that, entered a school race in the process and ran the 100m in 11.72 seconds. When his father looked up this time, he established that his 17-year-old son had run faster than the Paralympic record at the time.

Pistorius was taken to an athletics club and, just six months later, won gold in the 200m at the Athens Paralympics, breaking the world record. But it wasn’t enough. Pistorius, who had been used to competing against able-bodied runners, wanted to continue doing this and, in March 2005, at the South African championships, he finished sixth in the 400m final.

He went on to win golds in the 100m, 200m and 400m at the 2008 Paralympics in Beijing, the first athlete ever to do that. Last year he became the first Paralympian to win an able-bodied world championship medal, in the 4 x 400m relay at Daegu in South Korea.

“I never set out to be controversial,” he says. “I’m not trying to break down barriers. It’s much simpler than that: I’d always competed against able-bodied athletes and still wanted to be able to do that.”

But Pistorius has courted a great deal of controversy. He runs on prosthetics called Flex-Foot Cheetahs, which are made of carbon fibre and stand out because they look very unlike traditional false legs.

The Flex-Foot prosthetics are perfectly legal in Paralympic sport, but in 2007 his use of them in able-bodied races was challenged. The International Association of Athletics Federations filmed him and declared that the prosthetics conferred some advantage on him. Competition rules were changed, banning “any technical device that incorporates springs, wheels or any other element that provides a user with an advantage over another athlete not using such a device”.

Pistorius fought back, and at the Court of Arbitration for Sport in May 2008, it was announced that he had no such benefit over able-bodied athletes. From now on, runners with disabilities wishing to compete in able-bodied events will always be tested to make sure that their prosthetic limbs offer them no unfair advantage.

“I don’t want to feel disabled by the disabilities I have, but enabled by the abilities I have,” he says.

All he wants now is the freedom to compete in his sport at the highest level. “I’m an athlete,” he says. “I realise that I have a disability, but I also have lots to offer. I’ve been very lucky in so many ways, and I want the chance to give all I can for my family, my God and my country.”