Former world number one Andy Murray is in line to play Grigor Dimitrov in the opening round of the inaugural ATP Cup, with Bulgaria qualifying among the final six teams in the nation-based tournament.
Briton Murray, on the comeback trail after hip surgery, was initially set to face Roger Federer in a blockbuster Group C clash in Sydney but the 20-times Grand Slam champion pulled out of the January 3-12 tournament, making Switzerland ineligible.
Bulgaria have replaced Switzerland in the group, meaning Murray instead faces a prospective match with Dimitrov, dubbed "Baby Fed" for his similar playing style to the Swiss master.
Chile, Poland, Uruguay, Moldova and Norway are the other five teams that qualified for the 24-nation field at the close of the second and final deadline.
The six group winners and the two best second-placed finishers across the groups reach the ATP Cup Finals in Sydney.
Each tie comprises two singles and one doubles match.
World number one Rafa Nadal kicks off his tournament with Spain in Perth against Group B rivals Japan, Georgia and Uruguay, the tournament draw confirmed on Thursday.
Serbia, led by world number two Novak Djokovic, play Chile, France and South Africa in Group A in Brisbane.
Russian young gun Daniil Medvedev, beaten in the US Open final by Nadal, will highlight Group D for Russia against the United States, Italy and Norway.
Group F has Germany, Greece, Canada and Australia playing off in Brisbane and boasts a mouthwatering array of young talent.
German world number six Alexander Zverev is likely to feature in the singles along with Greek tyro Stefanos Tsitsipas and tempestuous Australian Nick Kyrgios.
Group A - Serbia, France, South Africa, Chile (Brisbane)
Group B - Spain, Japan, Georgia, Uruguay (Perth)
Group C – Belgium, Great Britain, Bulgaria, Moldova (Sydney)
Group D - Russia, Italy, United States, Norway (Perth)
Group E - Austria, Croatia, Argentina, Poland (Sydney)
Group F - Germany, Greece, Canada, Australia (Brisbane).
Bryan brothers to bow out after 2020 US Open
Bob and Mike Bryan, the greatest doubles pair in the history of tennis, have announced they will end what they describe as a "magical ride" in 2020 after one more crack at the US Open.
The 41-year-old Californian twins, famous for their trademark chest-bump celebration, will bid farewell at Flushing Meadows, scene of their Grand Slam debut in 1995.
Playing almost exclusively with each other throughout their 25-year careers, the Bryans captured a professional era record 118 titles, including 16 Grand Slam titles, 39 ATP Masters 1000s and the ATP Finals title four times.
The Bryans also won Olympic gold at London 2012 and were part of the US Davis Cup-winning team in 2007.
Mike claimed two more Grand Slam titles with Jack Sock in 2018 after Bob was sidelined with a hip injury.
"Mike and I chose to finish our 2019 season after the US Open, even knowing there was a strong chance we'd qualify for the ATP Finals," Bob said in an ATP statement on Wednesday.
"After much discussion, we decided that it would be best to rest our minds and strengthen our bodies in preparation for 2020 which will be our final season on the ATP Tour.
"For the last 21 years, we have been so grateful for the opportunity to live out our dreams of playing professional tennis. It has truly been a magical ride."