Andy Murray fought back to beat Kei Nishikori in an epic contest and close in on a semi-final place at the ATP World Tour Finals.
The world number one lost a tie-break but came through 6-7 (9-11) 6-4 6-4 against the Japanese fifth seed.
At three hours and 20 minutes it was the longest match since the event moved to London in 2009.
Murray, 29, will qualify for the last four if Marin Cilic beats Stan Wawrinka in the evening session at 20:00 GMT.
It is the first time since his debut in 2008 that the Briton has won his opening two group matches.