Thiem lost in straight sets to Roger Federer on Tuesday which means he is winless so far but he still has a mathematical chance of qualifying for the semi-finals.
The permutations make Thiem's progression highly unlikely though and he will find it difficult to pick himself up after a couple of disappointing displays.
The Austrian has managed to pick up a win against Nishikori this year but that was on clay where he is more comfortable.
Nishikori was completely destroyed 0-6, 1-6 by Kevin Anderson on Tuesday and was almost faced with the embarrassment of the double-bagel so he will need to pick himself up quickly after that mauling.
He has a better mathematical chance of qualifying than Thiem does so his motivation levels should be greater heading into to this final group stage match.
Nishikori certainly has the edge in career meetings having won three of their four previous matches but he knows he needs to improve his levels of performance after the Anderson defeat.
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. Having said that, he put in an abject display on Tuesday as he was completely destroyed by Kevin Anderson so that may have a psychological impact on his performance here.
Thiem has not showed his best tennis at the O2 and looks like he can't wait for the season to end, so that just about gives Nishikori the edge but it could get very nervy. Nishikori in three sets.