Tennis great Billie Jean King has admitted Ashleigh Barty's sudden retirement last year left her 'upset' and insisted players should not bow out while at the top of their sports.

Barty shocked the sporting world in March, walking away from tennis for a second time to 'chase other dreams' less than two months after winning the Australian Open. 

And the 79-year-old said she felt the Aussie star had ended her career far too soon, although she acknowledged Barty's personal happiness was the main concern.

'I was very upset she retired, she’s a great athlete but more importantly I do want her to be happy,' King told the Herald Sun.

'She’s so young, she’s great for the sport.'

At the time of her retirement, Barty had held the status of World No1 for since June 2019 and had just won a third Grand Slam title in three years.

King, however, insisted she did not agree with the accepted wisdom that athletes should retire while at their peak.

'I think people should not be encouraged to go out on top,' she continued.

'Bill Bradley, he was a basketball player, had a book out and I wish I’d read it before I retired.

'He talked about sport’s like a lifetime and you should play as long as you can … it’s like a life.'

A 12-time Grand Slam singles tournament winner, King walked away from tennis relatively early herself, retiring at the age of 32 shortly after winning her sixth Wimbledon crown in 1975.

She has since conceded the decision was a mistake and she returned to the court a few months later, before eventually going on to play for another 15 years.