Players put the tournament at the top end of their priority list. What is it that makes this it so special? The answer is the history that surrounds it.
Novak Djokovic is still the favourite to claim a 4th consecutive ATP World Tour Finals title according to Roger Federer, despite ending his winning streak.
Rafa Nadal's late-season resurgence continued at the ATP World Tour Finals when he beat wayward French Open champion Stanislas Wawrinka to open his account.
Andy Murray said he would put security concerns about next week's Davis Cup final in Belgium to the back of his mind despite several arrests near Brussels.
You won't have guessed Andy Murray spent the past week sliding on clay rather than honing his hardcourt skills in preparation for the ATP World Tour Finals.
Novak Djokovic trounced Kei Nishikori and Roger Federer demonstrated his swagger again when dismantling Tomas Berdych as the ATP World Tour Finals began.