Even now, three decades on, it is hard to understand.
A modern sporting parallel would be Wayne Rooney, Alastair Cook or Rafael Nadal, suddenly deciding enough was enough, hanging up their boots, bat or racket, and disappearing from sporting life.
In 1981, Bjorn Borg won the French Open and reached the finals of Wimbledon and the US Open. During the summer he had exchanged the world No 1 spot with John McEnroe. Yet after the latter beat him in New York, he walked off the court, not even waiting for the presentation, and into retirement. There were a couple of ill-fated comebacks but, to all intents and purposes, one of the world's great players had jacked it in, at the age of 25, with 11 Grand Slam titles.
During the intervening decades, the Swede went five years without picking up a racket and nearly 20 without visiting the Wimbledon lawns on which he made his name. He was persuaded back, as a one-off, for the Parade of Champions in 2000. Then, in 2007, he returned again. That seemed to trigger something, maybe provide some closure, because this year will be his fifth visit in succession. Suddenly he cannot keep away from the place.
"That's true," he says with a rueful grin, as we talk in, appropriately enough, Wimbledon's Champions Room. The famous blond mane lacks a headband and is now silver, but it remains lush. Lean and tanned, he is still recognisably the man who won five successive Wimbledons in 1976-80. The difference, from the Nordic ice-Borg of legend, is the warm smile and effusive manner.
The best explanation Borg can give for his sudden retirement is that he just lost motivation. He had been on the professional circuit from the age of 14 and dedicated himself to playing tennis as well as he humanly could. Without that drive there was no point continuing. It was all or nothing, and nothing meant forgetting about tennis, including SW19, and seeing what else life had to offer.
The answer was plenty, not all of it good. He was married and divorced twice, fathered a child with a teenager he met while she was competing in a wet T-shirt contest (he was a judge), and developed a business that collapsed into bankruptcy.
After needing to have his stomach pumped, he was even alleged to have attempted suicide, but he has always insisted it was an episode of food poisoning which was scandalously reported. "When I retired I tried other things in life," he said, "negative and positive, but just try to learn things about life. My life had been training, hotel, match, airport, training. To do other things is very difficult for someone who is not used to it. I had possibilities to do more things, but at first I was not really ready. I learned eventually a lot of things.
"You never get the same kick as playing tennis [at championships], but you have to find something. It is a learning process. After playing tennis I had years when I don't think about tennis. I didn't touch a racket for five years. I exercised every day as I always wanted to stay in good shape, but it was ice hockey, soccer or running. It was important to exercise every day. I hope I will live to 150 years, and always stay in good shape.
"All those years I played tennis, I have no regrets. All those things that teenagers do, that my friends were doing, I didn't do them. I didn't miss it. I was having a great time playing tennis. I sacrificed all that because I had my heart in tennis. I had my goals. I wanted to be best. I wanted to be successful, and I was very successful even if I retired at 25, 26. It is a young age. My coach, Lennart Bergelin, said, 'Come on, you have five more years, you can win more things'. I said, 'Yes, but if I am not motivated I cannot continue.'
"Up to 25, 26, I have no regrets. It was the best time of my life. I was successful, not just for myself but for Swedish tennis, international tennis. I did a lot of things, with McEnroe and [Jimmy] Connors. We lifted tennis to a different level, what happens now they have to thank us for. We made it global, with bigger stadiums, more money coming in. I'm proud of that."
It was not just the image of tennis that Borg, arguably the first male player to fixate both the groupies and the tabloids, transformed, it was the way the game was played.
"I was the first one really to work hard as a professional athlete, to spend many hours not only on the court, but also outside," said Borg, "I was the first who had a tennis coach, the first in many things in tennis."
"He was a leader in his time," said Tim Henman, the former British No 1 who said he was inspired to take up the game as a six-year-old after watching Borg play in his last Wimbledon. "Even with a wooden racket he was the first to come in with much more top-spin, the first with the two-handed backhand."
Henman added: "You watch the Borg-McEnroe final of 1980 now and it looks slow-motion, but they are playing with wooden rackets. I've tried that a few times and it's not easy. The racket head is so small and the sweet spot is tiny. It is a different technique. He was one of the greatest athletes that ever played this game. You give him his time again, and in this generation with the modern training techniques, and he would still have been a legend."
Henman, who played to 33, by which time injuries and age had dragged him down the rankings, said: "It is still amazing he won 11 majors and stopped at the age of 25, 26. I can't understand that, but everybody is different. Look at how demanding his game was, how much it took out of him, how much he achieved."
Borg hints that, had he played the modern game, he might have gone on longer. "I think players are more protected than we were. When we were staying in hotels there was like 100 people waiting in the lobby. Maybe that was why I stepped away – not the pressure of the tennis, it was the people, the mania."
He added: "But I am happy with what I did. I had my time, I had my success – it's not like I am sitting home scratching my head, thinking it's no good. I am not that person."
Borg's business, selling fashion clothes, notably underwear and beachwear, is successful again. His third marriage is approaching a decade and he lives happily in Stockholm. After appearing to have lived the first two stages of his life in the wrong order, a wayward adolescence having followed a driven career period, he seems to be enjoying a contented maturity.
This is reflected in his reappearance at Wimbledon. "After 1981 I didn't come back for many years but that was not to do with Wimbledon," he said. "I was not ready to come back. It is my favourite tournament, here started my success. Someone said a few years ago I have something against it – that was one of the most stupid things you'll hear. It has changed, but it still has the same atmosphere, it is the same ground."
HSBC, Official Banking Partner of Wimbledon, is giving fans the chance to vote for their favourite moments and win a chance to be at the Wimbledon Finals in 2012 – see www.wimbledon.com/hsbc
The shape of tennis to come: tougher and taller
Having been an innovator as a player, Borg was intrigued by the results of a study by Loughborough University, commissioned by HSBC to mark Wimbledon's 125th anniversary, into how tennis might be played in 2036, the 150th year.
Among the findings was the prospect of technology embedded in racket handles to measure players' heartbeat and sweat to calculate their stress levels. Borg, whose resting heartbeat is around 40, says: "Everybody wants to know everything these days, and people watching on TV will want to know, but would you want your opponent to know your level of fatigue?"
Further racket development to increase power is likely but again Borg cautions: "Everybody already hits the ball so hard all the time. With wood there were lots of different ways to play – it is a new game now. When you have someone who is tall, moves well, with great reach, and serves hard, that will be tough to play."
Tim Henman noted: "When I started on the tour, in the mid-Nineties, I was one of the taller players at 6ft 1in. By the time I finished, 10 years later, I was one of the shortest." Players continue to get taller, with Ivo Karlovic 6ft 10in and John Isner 6ft 9in, but the top four are all around 6ft 2in. Borg, like McEnroe, is 5ft 11in.