Novak Djokovic, Rafael Nadal and Dominic Thiem are the headline names who will be representing their nations at the ATP Cup in Melbourne next month.
Serbia defeated Spain in last year's final and the two could well face off again as the field is reduced to 12 countries from 24 as a result of the Covid-19 pandemic.
The draw is on January 20 with the tournament taking place from February 1-5.
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Great Britain have not qualified for the 2021 event as it is based on the ATP ranking of each country's top-ranked singles player with Dan Evans the highest-ranked at number 30.
Hosts Australia have been handed a wildcard.

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SERBIA: Novak Djokovic, Dusan Lajovic, Filip Krajinovic, Nikola Cacic
SPAIN: Rafael Nadal, Roberto Bautista Agut, Marcel Granollers, Pablo Carreno Busta
AUSTRIA: Dominic Thiem, Dennis Novak, Philipp Oswald, Tristan-Samuel Weissborn
RUSSIA: Daniil Medvedev, Andrey Rublev, Aslan Karatsev, Evgeny Dosnkoy
GREECE: Stefanos Tsitsipas, Michail Pervolarakis, Markos Kalovelonis, Petros Tsitsipas
GERMANY: Alexander Zverev, Jan-Lennard Struff, Kevin Krawietz, Andreas Mies
ARGENTINA: Diego Schwartzmann, Guido Pella, Horacio Zeballos, Maximo Gonzalez
ITALY: Matteo Berrettini, Fabio Fognini, Simone Bolleli, Andrea Vavassori
JAPAN: Kei Nishikori, Yoshihito Nishioka, Ben McLachlan, Toshihide Matsui
FRANCE: Gael Monfils, Benoit Paire, Nicolas Mahut, Edouard Roger-Vasselin
CANADA: Denis Shapovalov, Milos Raonic, Peter Polansky, Steven Dieg
AUSTRALIA: Alex de Minaur, John Millman, John Peers, Luke Saville
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