World No1 Novak Djokovic has withdrawn from the ATP Cup, raising doubts over whether he will compete in next month’s Australian Open.
The ATP Cup begins on Saturday in Sydney and will see 16 countries take part, including Djokovic’s Serbia.
This follows the withdrawal of Austrian dup Dominic Thiem and Dennis Novak.
Djokovic does however remain on the entry list for the Australian Open.
The Grand Slam is the first of the season and organisers have said competitors need to either be vaccinated or have a granted medical exemption.
The 34-year-old Serbian has not publicly stated whether he has had a Covid-19 vaccination or not.
He won the Open last year, beating Daniil Medvedev in the final, on his way to a season which saw him win his 20th Grand Slam title.
Djokovic, Rafael Nadal and Roger Federer are all tied on 20 titles.
Great Britain have been drawn to face Germany, Canada and the US in the opening group stages of the tournament.
GB will be led by British No1 Cameron Norrie, who won the coveted Indian Wells title last season, while Dan Evans, Liam Broady, Joe salisbury and Jamie Murray will also feature.
Only the group winners qualify for the semi-finals and group ties will be the best of three matches – two singles and one doubles match.
Russia won the title in 2021, beating Italy, while Serbia won the first edition in 2020.