Given the fear that can so often accompany the exit from the comforting bubble of doing what they have always done best, what is it that persuades top sportsmen and women to call it a day?

Three of the most obvious reasons can be summarised in the experiences that prompted three of the greatest footballers ever to have laced boots to reach that sad but inevitable conclusion.

When Stanley Matthews retired from football in 1965 he was known all over the world for his sportsmanship and skill. 

He had been persuaded to make it his last season for the reason that he was, as he recalls in his autobiography The Way It Was, "beginning to pick up injuries and even the niggly ones which years ago I would have shaken off in a day were taking two or three weeks to dispel".

Hardly a big surprise, as Matthews was 50 at that time. Even the Wizard of the Dribble could not evade the quintessential experience of the old pro.

For Bobby Charlton, the end of his playing days took a different shape.

In his autobiography My Manchester United Years, written in collaboration with my late lamented colleague James Lawton, Charlton recalls the day in 1973 when he knew it was time to end his playing career.

The Englishman, renowned all over the globe and a World Cup, European Cup, league championship and FA Cup winner with more than 100 caps for his country, was already aware he was "reaching the point when Saturday morning was no longer the greatest part of the week".

His conviction crystalised during a match against Birmingham City. "I couldn't remember ever running so hard and so long," he recalled. "I chased and chased, but there was nothing there for me, we lost the match and, I do not think it is too dramatic to say, I lost that last belief that, for a while at least, I could still be a United player."

Waning physical ability is another obvious reason why top-class players and athletes retire. But there are other, equally cogent considerations - such as a sense of what is fitting and well-timed.

Like Charlton, Pelé - three times a World Cup winner with Brazil - was still operating at a high level when he called an end to his own stellar career after helping New York Cosmos win the North American Soccer League title in 1977.

He was 37, and still fit enough to play on.

But, as he wrote in his autobiography Pele, "I quit because I was doing what I had always done…I had become champion at Santos, in the army team, and for Brazil. Now I was champion at the Cosmos too. It was time to leave."

Injury. Diminishing powers. Getting out at the top. Classic call-it-a-day reasons. But now, in a unique and unwelcome twist, we have another - courtesy of the COVID pandemic.

The impact coronavirus has had upon the sporting calendar - most notably the postponement of the Tokyo Olympics and Paralympics from this summer to next - has tested the will of many elite performers to the limits. And in some cases, broken their resolve to carry on.

The effect has been most dramatically felt by those who had been targeting the Tokyo Games as a fitting finale to their careers.

China's Lin Dan, at 37, was contemplating seeking a third Olympic badminton singles title at the Tokyo Games. But extending that ambition to next summer proved a bridge too far for the man known as "Super Dan", and he announced his retirement earlier this year.

Similarly, New Zealand cyclist Eddie Dawkins, a three-time world champion and Rio 2016 Olympic silver medallist, gave up his plans to seek an Olympic finale in the men's sprint team when the Tokyo Games were rescheduled, commenting: "To now face a further year on top of that, and with no certainty even then, is a step too far for me."

British rower Tom Ransley, 34, a member of the winning eight at the Rio 2016 Games, also decided an extra year would be too much of a stretch, while New Zealand hockey player Gemma McCaw, married to All Black Richie, decided not to pursue what would have been a fourth Olympic appearance in the Japanese capital.

Australian basketball player Andrew Bogut, 36, who had planned to compete in his fourth Olympics in Tokyo and seek a place on the podium having finished fourth at the Rio Games, is another who has found the hiatus too demanding.

Bogut conceded he would need to take "a lot of painkillers" to get to Tokyo 2020, but felt it was "just not worth it", claiming his body was "hanging by a thread".

Australian hockey co-captain Jodie Kenny, 33, decided the lure of trying to earn an Olympic medal at the third attempt was not enough to enable her to do what she knew was required to be in top shape for next summer.

"Making this decision to retire was a mix of everything," said Kenny.

"Emotions, motivation and uncertainty around what next year will look like with COVID and the extra commitments around travel and quarantine periods.

"My heart was still wanting to go on and play but my head just wasn't anymore."

Others at the top of their sport have deliberated whether to go the extra mile to the shifted Games - and no-one has done so with more intensity than the multiple Olympic and world gymnastics gold medallist Simone Biles, who made her ambivalence very clear before finally opting to extend her career until the other side of Tokyo.

The 23-year-old from Columbus, Ohio was one of many to suffer at the hands of disgraced team doctor Larry Nassar, and has been a strong critic of what she sees as the inadequate preventative and restorative measures taken by USA Gymnastics.

In tennis, identical twins Bob and Mike Bryan. who have won multiple Olympic medals in men’s doubles, including the gold in 2012, and have won more Grand Slams than any other men's pairing, decided In August that they would retire when what they had hoped would be a farewell tour of their favoured courts turned into a prospect of empty stands and remote viewing.

More recently Julia Görges, one of Germany's leading players, made an unexpected announcement of her retirement.

Aged 32, she decided that life outside her sport, which the pandemic lockdown obliged her to experience more fully than she had for many years, was calling her to new experiences.

"It just showed me how nice life can be away from tennis, too," she said. "If you just have it for two to three weeks like in the off-season, [retirement] never comes to your mind, but if you have stayed away from the tour for five to six months, it just opens up a whole different new chapter."