Djokovic will look to underline his status as the best player in the world and lifting the title here would be a perfect end to an extraordinary year in which the Serb has returned to the top of the game. His confidence is soaring and his recent win over Zverev in the round-robin will only serve to increase that.
Federer has improved as the tournament has progressed and will go into this as a slight favourite, although he will not be expecting an easy ride against Zverev as the pair have shared some tough battles in the past.
Cilic will need his serve to be hitting the lines with regularity here if he is to challenge Djokovic, but given the largely one-sided nature of their rivalry and the current irresistible form of the world number one, a win for the Croat seems unlikely.
Zverev will not be too disheartened by his loss to Djokovic on Wednesday as before the tournament he will have been targeting wins against Cilic and Isner.
Federer will still be smarting after his defeat to Anderson in the Wimbledon quarter-finals so he will be out for revenge here, as well as a spot in the semi-finals. The Swiss looked more like his usual self against Thiem on Tuesday and now appears to have found his rhythm on court, and he will need every ounce of his talent to overcome an in-form and confident Anderson.
Although both players can still technically progress, it is Nishikori who has put himself in a better position to qualify from the group so he should have the edge in motivation here.
Two big servers face off here in a match that is vital to the pair's qualification hopes after opening match defeats. Cilic will probably feel he should have beaten Zverev on Sunday but his inability to convert chances in important moments plagued him yet again and it will be interesting to see how he responds here.
Djokovic is at the peak of his powers at the moment and it would take something extraordinary to stop him if he plays to the levels he showed against John Isner on Monday. Zverev represents an entirely different opponent and it is likely to be more of a slugfest from the baseline, but Djokovic's metronomic consistency makes him more than up to the task of such an attritional battle.
This promises to be an intriguing match-up between two men desperate to atone for defeats in their respective openers. On the basis of those matches alone, it would be tempting to back Thiem here given that he wasn't as below-par as Federer was.
Two wins in the round-robin stage is often enough to qualify for the semi-finals so Anderson and Nishikori will know the importance of this clash after they were both victorious in the opening matches on Sunday.
Djokovic has largely been invincible in recent months and it is difficult to look past him winning the whole tournament, let alone this opener against an opponent he has beaten eight times in ten matches.
Both players will be looking to get out of the group for the first time in their career when they face off in what will be a repeat of their opening match at the O2 last year. On that occasion, Zverev won in three sets and a repeat scoreline here would not be a huge surprise.
Two recent defeats to Federer will be fresh in Nishikori's mind but he did compete well in their Shanghai and Paris matches so he certainly has what it takes to test the six-time champion.
This tournament opener is a difficult match to call as although Anderson has mostly had the upper hand in this particular rivalry, it is the Austrian who has won both their meetings in 2018.