Last year's runnner-up Dominic Thiem will miss the Australian Open this year as he continues his recovery from a serious wrist injury.
The 28-year-old Austrian has not played since a second-round defeat in Mallorca in June, meaning he was unable to defend his US Open title in September.
There also remains significant doubt about world number one and and nine-time Novak Djokovic's participation as he remains tight-lipped about his Covid-19 vaccination status.
Rafael Nadal, Andrey Rublev and Denis Shapovalov are all recovering after testing positive for the virus in recent weeks.
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