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Novak Djokovic withdrew from the ATP Cup on Wednesday, furthering speculation of the world No. 1 also missing the upcoming Australian Open.  The ATP declined to give a reason for Djokovic's withdrawal, and the 20-time Grand Slam winner is not dealing with any known injuries. 

The ATP Cup will begin Jan. 1 in Sydney, just 16 days before the Australian Open's start in Melbourne. Dusan Lajovic will replace Djokovic on Serbia's roster for the ATP Cup, a team event featuring 16 countries. 

Djokovic's COVID-19 vaccination status could very well have factored into his decision. The ATP Cup didn't establish a vaccination mandate, but the Australian Open is requiring every player get jabbed -- or a medical exemption -- to compete. Djokovic has repeatedly refused to say whether he's vaccinated. 

While speaking to Melbourne media last week, Australian Open director Craig Tiley was adamant his tournament won't allow unvaccinated players' participation -- regardless of their star status. 

"If Novak shows up at the Australian Open, he'll either be vaccinated or he'll have a medical exemption," Tiley said, per The Guardian.

Djokovic, the defending Australian Open champion, is on the tournament's entry list despite doubts of his participation. He's won the Grand Slam event nine times including last year. 

In late November, Srdjan Djokovic, Novak's father, told Serb TV his son would "probably" miss his chance to defend his Australian Open title over the tournament's vaccination mandate. 

"Under these blackmails and conditions, he probably won't [play]. I wouldn't do that. And he's my son, so you decide for yourself," Srdjan Djokovic said, according to BBC Sport. "... As far as vaccines and non-vaccines are concerned, it is the personal right of each of us whether we will be vaccinated or not. ... No one has the right to enter into our intimacy."

Djokovic's 20 Grand Slam singles titles are tied with Rafael Nadal and Roger Federer for the most ever. All three players are skipping the Australian Open, the latter two with injuries.