World number one Novak Djokovic has withdrawn from Serbia's ATP Cup team.

There remains uncertainty over whether the 34-year-old will compete at the Australian Open, which begins in Melbourne on 17 January.

All players and staff at the 2022 Australian Open must be vaccinated or have a medical exemption granted by an independent panel of experts.

However, defending champion Djokovic has not spoken publicly about his vaccination status.

The nine-time Australian Open winner has been replaced by world number 33 Dusan Lajovic for Serbia in the 16-country ATP Cup, which begins in Sydney on Saturday.

Austria have been replaced by France at the event following the withdrawals of Dominic Thiem, ranked 15th, and Dennis Novak.

Djokovic, who defeated Russia's Daniil Medvedev in 2021 final to claim a record-extending ninth Australian Open men's singles title, is on the entry list for the first Grand Slam of 2022.

Meanwhile, former US Open champion Sam Stosur has announced that the Australian Open will be the final tournament of her singles career.

The 37-year-old Australian will continue to play doubles in 2022 following what will be her 20th Australian Open and 69th Grand Slam singles appearance.

"It really means a lot to me to be able to finish in front of my family, friends and Aussie fans who have stuck by me through thick and thin," Stosur said.

"I'm sure it's going to be a mix of emotions but I wouldn't want it any other way as I always thought I'd finish playing in Australia. I do however plan to play the rest of the year in doubles but this will close the first chapter of my career."