Top seed Djokovic returned to the top of the world rankings following Rafael Nadal's withdrawal from last week's Paris Masters due to injury.
Nadal confirmed Monday he would skip the end-of-season finals in London to undergo ankle surgery. American Isner, the lowest seed in the eight-player tournament, was confirmed as the Spaniard's replacement. No. 3 seed Zverev and No. 5 seed Cilic complete Group Guga Kuerten.
Roger Federer, seeking his 100th career ATP title and a sixth end-of-year crown, is the No. 2 seed at the O2 Arena and has been drawn alongside No. 4 Kevin Anderson, No. 6 Dominic Thiem and No. 7 Kei Nishikori in Group Lleyton Hewitt. Anderson came from two sets down to defeat Federer in their Wimbledon quarterfinal in July.
The ATP Finals take place in London on Nov. 11-18, with Group Hewitt matches kicking off the action on Sunday.