Novak Djokovic has withdrawn from the Serbian team taking part at the ATP Cup from 1 January in Sydney, throwing his participation in the Australian Open further into doubt. Organisers have confirmed Djokovic won’t be competing in Sydney, with the Serbs now to be led by world No.33 Dusan Lajovic. The world No 1 would be bidding for a men’s record 21st grand slam title and a 10th Australian Open crown in Melbourne. The 34-year-old is on the entry list, but yet to commit to playing after the Covid-19 vaccination mandate agreed by Tennis Australia and the Victorian Department of Health. Djokovic has repeatedly declined to disclose whether or not he is vaccinated, citing privacy concerns. The Australian Open chief Craig Tiley said last week he was still uncertain whether Djokovic would be playing at the Melbourne Park major, which begins on 17 January. All players and staff at the Australian Open will be vaccinated or have a medical exemption granted by an independent panel of experts. Those requirements have prevented Djokovic from confirming whether he will bid for a men’s record 21st grand slam title and a 10th Australian Open crown. Djokovic is the most high-profile of a string of stars who have either pulled out or are doubtful for the Open. Beaten 2020 finalist Dominic Thiem withdrew on Tuesday night, citing a wrist injury, joining 20-time grand slam winner Roger Federer (knee) on the injury list. Fellow 20-time title winner Rafael Nadal, current world No 5 Andrey Rublev and Denis Shapovalov (No 14) have contracted the coronavirus and may be doubtful participants. On the women’s side, Serena Williams, Karolina Pliskova and Bianca Andreescu won’t be in Melbourne while Olympic champion Belinda Bencic and US Open winner Emma Raducanu have contracted the coronavirus.