Kevin Anderson makes his debut at the year-ending finals having risen to number six in the world after a career-best season which saw him pick up two titles and reach the Wimbledon final.
The big-serving South African most recently reached the Round of 16 in the Paris Masters before losing to Kei Nishikori, but prior to that he beat the same opponent in the final of an ATP 500 event in Austria to collect his fifth career title.
Anderson has a 6-2 lead in career meetings with Thiem with all but one of those matches coming on hard courts so he knows what it takes to overcome the Austrian.
World number eight Dominic Thiem is appearing in the finals for the third straight year but he has previously been knocked out in the round-robin stage on both occasions so he will look to improve on that record at the very least.
The Austrian has been fairly consistent without being spectacular on the recent hard court stretch and reached the semi-final of the Paris Masters before putting in a disappointing performance in losing to Karen Khachanov.
Despite a losing head-to-head against Anderson, Thiem will take heart from the fact he has won their last two meetings - both of which were this year - with their most recent being an impressive straight sets victory at the US Open.
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.
Anderson will need his serve to be on point throughout this match to penetrate his opponent's excellent defence, while Thiem's serve is a more formidable weapon than it's given credit for so it would not be a surprise to see a close content settled on a couple of tie-breaks. Recent form just about gives Thiem the edge and his return game could be key in breaking down his South African opponent's chief weapon.
Thiem will be motivated to improve upon his record at the O2 and has a good chance of getting his campaign off to a solid start. Thiem in three sets.