Serena Williams insists she remains "motivated" to keep playing tennis despite her fast-approaching 41st birthday and the absence of a  Grand Slam title since 2017.

After her latest Wimbledon setback, should the 23-time major winner retire and preserve her legacy?

AFP Sport looks at three stars who called it quits too late.

Muhammad Ali

Arguably the greatest heavyweight boxer of all time and comfortably one of the 20th century's most influential sportsmen, Muhammad Ali won the world title for the first time in 1964 at the age of 22 with a sensational defeat of Sonny Liston.

His fights against Joe Frazier and George Foreman are legendary landmark moments.

In 1978, he was stunned by an unknown Leon Spinks before avenging the defeat later that year to become the first fighter to claim the world heavyweight title on three separate occasions.

Ali retired, quickly changed his mind and agreed to fight Larry Holmes in Las Vegas in 1980.

Holmes was untroubled by an out-of-shape Ali, who was past his best. "Rocky" actor Sylvester Stallone described the ugly spectacle as "watching an autopsy on a man who is still alive."

For the first time in his career, Ali lost on a stoppage.

Despite calls to retire, Ali fought again in 1981 at the age of 39 against a 27-year-old Trevor Berbick before a series of farcical exhibitions damaged his legacy.

Ali was estimated to have taken nearly 200,000 blows in his career and was diagnosed with Parkinson's disease in 1984. He died in 2016 at the age of 74.

George Best

Manchester United footballer George Best's extravagant skills on the pitch were matched only by his colourful adventures off it.

With his long hair and rock star looks, the Northern Ireland winger was even dubbed the "Fifth Beatle" and helped United to the European Cup in 1968 as well as two English league titles.

In his Old Trafford career, he scored 179 goals in 470 appearances.

A European footballer of the year, Best was plagued by personal demons, most notably a lengthy battle with alcohol. 

After leaving United in 1974, Best drifted from club to club, often failing to appear for training.

He turned out for Jewish Guild in South Africa and Los Angeles Aztecs, San Jose Earthquakes and Fort Lauderdale Strikers in the United States.

He even had a handful of appearances for two teams in Hong Kong and a spell with Hibs in Scotland.

Best died at the age of 59 in 2005.

Michael Schumacher

One of the greatest Formula One drivers of all time, Michael Schumacher won seven world championships between 1994 and 2004, a figure since matched by Lewis Hamilton.

The German drove for Bennetton and Ferrari in his prime.

In all he collected 91 wins, 68 pole positions and 155 podium places, all records at the time but since surpassed, again by Hamilton.

He announced his retirement in 2006 but in his last race in Brazil he took his Ferrari from 19th place to a fourth-place finish.

Media reports raved over what one called "an utterly breathtaking drive".

However, in 2010, Schumacher returned to the sport, despite being 41 years old, with a Mercedes team returning to the grid for the first time since 1955.

He managed only a ninth-place finish and ended the season without a win, pole position, podium or fastest lap for the first time since 1991.

In 2011 he was eighth in the championship while in his final year he was 13th, before it was announced that Hamilton would join the team the following season.

In December 2013, Schumacher suffered a severe brain injury in a skiing accident and has not been seen in public since.