Nishikori has been drawn into "Group Lleyton Hewitt" alongside Federer, Kevin Anderson and Dominic Thiem.
The two men, with 34 Grand Slams between them, are the twin titans at the season-ending event at London's O2 arena.
Despite the popularity of the London venue with fans and players alike, the ATP announced in August that it was inviting bids from other host cities for 2021 onwards.
Djokovic is playing in the group named after Brazilian player Guga Kuerten and is joined by Germany's Alexander Zverev, Croatia's Marin Čilić and Isner.
The draw of the two groups maybe smiled at Federer. The tournament gets underway with Wimbledon runner up Kevin Anderson taking on French Open finalist Dominic Thiem. Nishikori was also denied a victory in front of his home support at the Japan Open, after beating the likes of Stefanos Tsitsipas and Richard Gasquet en route to the final, where he fell to Russian Daniil Medvedev.
Speaking on The Jonathan Ross Show on Saturday night, he said: 'It's been a great season, especially the last five months.
Conjoined twins: Australian surgeons try to separate Bhutanese girls
Crameri said one of the risks was the use of anaesthetic, as they did not know how one twin would react to the other receiving it. A post on the charity's blog said that the girls' mother, Bhumchu, has seen her girls post-surgery and given them each a kiss.
He will be presented with the ATP World No. 1 Award at the O2 on Sunday, but Djokovic is by no means done yet - and the Serb confessed he has his sights firmly set on surpassing Federer and Rafael Nadal's total of Grand Slam titles. Thereal question is: can he beat Nole?
Last five tournaments: Paris Masters - quarter-finals, Swiss Indoors Basel - semi-finals, Shanghai Masters - semi-finals, China Open - last 16, US Open - last 32. The Japanese will be playing in the tournament for a fourth time after an encouraging season, though he has not won a tournament since 2016. Zverev is keen to speed up play and is fed up with opponents who take ages wiping themselves with a towel.
The German down coincides, like a year ago, with the last part of the season. The most serious rival in his round robin is Cilic.
They shared the stage for one of the most memorable matches of the season when Anderson defeated Isner 26-24 in the fifth set of a six-hour-plus semi-final that directly led to Wimbledon announcing the introduction of final-set tie-breaks.
However, the gap between him and Djokovic is too wide.
The Austrian saved three match points in the tie-break, first when Anderson netted a forehand return, then by delivering a delicate forehand and finally an ace. Hard for Thiem. The Austrian, like Zverev, is not experiencing a good moment of physical shape, and the surface does not favor his tennis. Federer, who has won the tournament seven times, will be the star attraction on the opening day.