Andy Murray will try to reach the ATP World Tour Finals semi-finals for the first time in four years when he takes on Japan's Kei Nishikori on Wednesday.
The pair meet in the John McEnroe Group at 14:00 GMT, live on BBC Two, online and BBC Radio 5 live sports extra.
Britain's Murray is also aiming to stay ahead of Serb Novak Djokovic in the contest to be the year-end number one.
Croatia's Marin Cilic plays Swiss US Open champion Stan Wawrinka in the day's second singles match at 20:00.
The top two players in the group after Friday's final round-robin matches will progress to the last four.