Top-ranked Rafael Nadal and Ashleigh Barty lead the field at the 115th edition of the Australian Open from January 20-February 7.
Rafael Nadal put the finishing touches to his extraordinary year by clinching a sixth Davis Cup title for Spain on Sunday.
Rafael Nadal teamed up with Feliciano Lopez to secure a dramatic last-four victory over Britain in Madrid.
Rafael Nadal thrashed Diego Schwartzman 6-1, 6-2 and then teamed with Marcel Granollers to beat Maximo Gonzalez and Leonardo Mayer 6-4, 4-6, 6-3 in a must-win doubles match.
Rafael Nadal won his 26th consecutive Davis Cup singles match later on Wednesday as Spain booked their place in the quarter-finals with a 3-0 victory over defending champions Croatia.
Rafael Nadal's straight-sets win over Karen Khachanov helped Spain beat Russia 2-1 in the Davis Cup.
Rafael Nadal draws level with rivals Novak Djokovic and Roger Federer, who have also finished the year in top spot five times, and moves one behind six-time end-of-year number one Pete Sampras.
Novak Djokovic will be part of the Serbian Davis Cup squad which will participate in the tournament that is being played in a new format.
Dominic Thiem saw off the big-serving German 7-5, 6-3 after Greece's Stefanos Tsitsipas, making his ATP Finals debut, had earlier beaten Roger Federer.
Rafael Nadal exited the ATP Finals but ended his season as the world number one and with a chance to lead Spain to Davis Cup glory.
Rafael Nadal has never won the end-of-season tournament and had a miserable start at the O2 Arena, being outplayed by Germany's Alexander Zverev.
Defeat for Novak Djokovic spells the end of his bid to overtake Rafael Nadal and finish as year-end number one.
Matteo Berrettini arrived in London at a career-high number eight in the ATP rankings after starting the season outside of the top 50.
Rafael Nadal is on a double mission in London - aiming to win the end-of-season event for the first time in his illustrious career and to secure the year-end number one spot.
Rafael Nadal refused to make excuses after being outplayed by defending champion Alexander Zverev at the ATP Finals as Stefanos Tsitsipas got his campaign off to a flying start.
ATP Finals: Stefanos Tsitsipas registered his first-ever victory over Russia's Daniil Medvedev on Monday.
The "Big Three" remain the men to beat at the season-ending ATP Finals in London.
Rafael Nadal, who has never won the ATP Finals despite his trophy-laden career, dethroned Novak Djokovic from the top of the rankings this week.