Debutant Kevin Anderson made a confident start to his ATP Finals campaign with a straight-set win over Dominic Thiem in their group opener.
The South African world number six beat the Austrian 6-3 7-6 (12-10) at the season-ending event in London.
The match's only break came in the fourth game of the opener with Anderson then taking the second-set tie-break with an ace on his fourth match point.
Roger Federer faces Kei Nishikori later, live on the BBC at 20:00 GMT.
The ATP Finals features the world's top eight players - minus world number two Rafael Nadal and number four Juan Martin del Potro, who are injured.