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.
Qualification is based on the ATP ranking of each country's top-ranked singles player, while hosts Australia have been given a wildcard.
Last year's tournament featured 24 nations and Britain qualified through Andy Murray's protected ranking. They miss out this year, though, because Dan Evans is their highest-ranked player at world number 30.
The draw will take place on 20 January, when the 12 teams will be divided into four groups of three. The group winners will advance to the semi-finals.
The event has been moved to Melbourne - after being played in Brisbane, Perth and Sydney last year - to allow the players to stay in a Covid-safe bubble before the Australian Open starts on 8 February.
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