Novak Djokovic and Rafael Nadal will head the field at next month's ATP Cup, which has been reduced to 12 countries because of the coronavirus pandemic.
World number one Djokovic leads Serbia as they look to defend the title between 1-5 February in Melbourne.
Nadal, ranked second, represents a Spain side aiming to avenge their defeat by Serbia in last year's final.
US Open champion Dominic Thiem leads Austria, while ATP Finals champion Daniil Medvedev plays for Russia.
Great Britain, who agonisingly missed out on the semi-finals last year, have not qualified for the 2021 event.