It clears up an awkward issue for Djokovic at the start of a week which will see him try to confirm his rediscovered dominance, and prevent Roger Federer concluding the season with what would be a 100th ATP Tour title.
Usually all top 8 players participate, but No.2 seed Rafael Nadal and No.4 seed Juan Martin del Potro pulled out due to injuries. It has been a momentous year for the 32-year-old Anderson, who reached a career high of World No 5 in July and has become the first South African since Wayne Ferreira in 1995 to play in the season finale.
Federer will lead the charge in Group Lleyton Hewitt, which also consists of Dominic Thiem, Kevin Anderson and Kei Nishikori.
Nishikori opens against world No. 3 Roger Federer and knows it's a tough assignment after two recent losses against the Swiss star.
The 37-year-old has won a record six titles at the ATP Finals - including two in London - but has not lifted the trophy since 2011. "I'm going to compete thinking I've been given a bonus", said Nishikori, who had a one-hour training session with fifth-ranked German Alexander Zverev before the press conference. "I won that tournament, here I played the semis and I needed someone of Novak's calibre to beat me, so that's all right".
"But there was always part of me that believed I could make it back and I never thought it was impossible". He went on to reach the semifinals at the U.S. Open, where he lost to Novak Djokovic. "I'll play with confidence", he said. "It just shows you how good Connors was for so long and how much of a great competitor he was", said Darren Cahill, the ESPN analyst who coached the former men's No. 1 Andre Agassi and now coaches the women's No. 1, Simona Halep.
"It turned to out to be a ideal five months of the year, with two Grand Slam titles", he added.
Its longest stay in a single venue was across 13 consecutive editions at Madison Square Garden from 1977 to 1989 but it has been held at London's O2 arena since 2009 - drawing more than 250,000 fans annually. Bulgarian Dimitrov, who climbed to a career-high No. 3 after winning in London previous year, hasn't even made it, slipping to No. 19 in the world.
Zverev, 21, has been touted about as one of the leaders of the new generation and has won three Masters titles in his short career but he has flattered to deceive at the Grand Slams.
But it is hard to see anybody other than Federer or Djokovic prevailing.