The ruck was set just outside the Scotland 22.

A pile of bodies on the deck, a posse of defenders round the fringes. Nothing to do but shovel the ball on, churn through the phases, wait for a chance to open up. That was the script back then. That's what the ordinary player would do.

But Joost van der Westhuizen was no ordinary player. Not by any measure. The day before, the South African had flashed his angry, piercing eyes when the Murrayfield groundsman told him to stop practising on the international pitch and decided there and then that this would be his day. He would show the jobsworth something.

And how. The then 23-year-old scrum-half ripped the ball out, made as if to pass, then vaulted clean over the ruck. He ducked under an outstretched arm, shook off a tackle attempt, brushed past the last defender and raced away for the try, his second of the day.

This extraordinary score in November 1994 was also an early declaration of the genius that would shimmer so brilliantly in the South African sun six months later as the Springboks won a tumultuous World Cup.

Watch enough games of rugby, in enough places and over enough time, and you will see some special individual performances. I think of Dan Carter ripping the 2005 Lions to shreds; Jonny Wilkinson's mesmerising return for England against Scotland in 2007. I think of Fabien Galthie against England, Paris 2002; Gary Armstrong against Ireland, Dublin 1994; Dean Richards against Scotland, Murrayfield 1996. And I still think Van der Westhuizen's display that day 19 years ago was the best I've ever seen.

And now this. He is sitting in his wheelchair in a bar in Edinburgh's Fountainbridge. Those famous dark-rimmed eyes are unmistakable, but the look of pride and disdain they once expressed has gone. So, too, the body into which Van der Westhuizen once packed so much power His hands, almost useless now, are folded across his lap. He looks hollowed out.

All this we knew already. In 2011 Van der Westhuizen was diagnosed with the degenerative neurological condition motor neurone disease (MND) which occurs when specialist nerve cells in the brain and spinal cord called that control important muscle activity stop functioning properly. Most patients die within five years of diagnosis.

But knowing the facts of Van der Westhuizen's illness is one thing; seeing its effects in the flesh is another. He was big for a scrum-half at 6ft 1in but there was a time when the body that is failing him so pitilessly now seemed insufficient to contain all the energy and ability he had. With his combination of physical prowess and technical ability it is no exaggeration to say that he was, for perhaps three or four years, quite simply the best rugby player on Earth.

He admits he struggled to accept what was happening to him when MND began to take its savage toll. "You can imagine that if someone gives you a death sentence when you have a young family - kids of five and seven - then you are going to feel pretty bad," he said. "It took me about a year and a half to come to terms with it.

"But then I just woke up one morning I decided I would lead the rest of my life positively. I don't want to be remembered by my family as being a negative person who moaned about life. I want to be happy and I want them to be happy."

Van der Westhuizen's words are heavily slurred as he struggles to make recognisable sounds with a tongue that simply doesn't do what it is told any more.

His minders warn he tires easily these days, but he carries on. His time is limited and he is determined to put it to good use. "I want to be a better person," he says. "There were times in my career when people said I was arrogant, I only cared about myself. Now I know it is really about giving and caring for others."

I CAN vouch for the old image. I interviewed Van der Westhuizen in 1999 when he was captain of the Springboks side that had come to Edinburgh to defend their World Cup. He made no attempt to disguise his belief that the interview was an obligation, nothing more. There was no small talk, no warmth, just those cold, suspicious eyes.

There is a softness in them now, and he smiles easily and often. It is as if he has come to recognise its importance in needing it from others."I surround myself with good people, positive people," he says. "My two kids keep me up and give me pleasure. I have decided I am going to give them a dad for as long as possible. To do that I have to be very strong.

"It is difficult. When I told my son I was coming here to see a doctor he said, 'daddy, when you come back will you be better? I want you to play with me'. My daughter really wants me to pick her up and hug her, and that is hard."

Van der Westhuizen began fundraising, initially to help patients cope with the daily demands of living with MND, now through a link with Edinburgh University's Euan MacDonald Centre for Motor Neurone Disease Research.

A couple of hours before meeting him, I had asked South Africa coach Heyneke Meyer about him. Meyer was Van der Westhuizen's coach at Pretoria University and, later, at the Blue Bulls. A thoughtful, intelligent man, Meyer admitted what has happened to his former player has had a powerful effect on him and on South Africa as a whole.

"You almost think things don't happen to people like that," Meyer said. "It was a huge shock, we are very close. It just shows you have to live life to the full. You have to make use of every single second out there and enjoy life because there are a lot of other guys who can't. He is still fighting and my prayers go out to him."

Van der Westuizen's explanation for that is simple. "I have to fight, not just for me but for all the other sufferers," he says. "In South Africa there has been no research; the disease is hidden. People have had no information, so I decided I would do what I could to help them. That keeps me going and it makes me happy to see some benefits.

"I know how hard it is on families so we help them financially and with communications and information. They don't have to worry about anything because we are there for them, even with counselling and emotional support."

Of course, the wider rugby community has embraced him, as is its way. Scott Hastings ("an incredible man," says Van der Westhuizen) has organised many of the events of his current British visit, including a fundraiser at Murrayfield last Friday.

He will return for today's match. "I wasn't born to be a spectator," Van der Westhuizen says, but he can at least savour one memory from almost two decades ago. "I remember the groundsman well," he says. "When he chased me off, I thought, 'right, tomorrow I will show you'. So I scored the two tries and as I walked off after the game I saw him standing on the touchline, and I smiled at him. He just shook his head. It was a good moment."