SYDNEY—Novak Djokovic has pulled out of the Serbia team for the ATP Cup in Sydney, event organizers said on Wednesday, raising further doubts over whether the world number one 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. Organizers of the Australian Open, which starts on Jan. 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 “Novak Djokovic has officially withdrawn from the ATP Cup in Sydney, we have no update as yet on his plans for the Australian Open,” a Tennis Australia spokeswoman said in an emailed statement. “Regarding the medical exemption process, which pertains to all players, there is a rigorous and …