Novak Djokovic has pulled out of the Serbia team for the ATP Cup in Sydney, event organisers said on Wednesday, raising further doubts over whether the world No.1 will defend his Australian Open title next month.

There has been intense speculation over Djokovic’s participation in both events, with the 34-year-old, who is opposed to Covid-19 vaccinations, repeatedly declining to say if he has been inoculated against the virus.

Organisers of the Australian Open, which starts on January 17, have stipulated that all participants must be vaccinated against the coronavirus or have a medical exemption granted by an independent panel of experts.

“Novak Djokovic has officially withdrawn from the ATP Cup in Sydney, we have no update as yet on his plans for the Australian Open,” a Tennis Australia spokesperson said in an emailed statement. “Regarding the medical exemption process, which pertains to all players, there is a rigorous and independent process in place. As with all medical information it is subject to confidentiality.”

ATP Cup organisers said Serbia’s team would now be led by world No. 33 Dusan Lajovic. Owing to Djokovic’s current world ranking, Serbia had received top billing for the 16-team event.