The group stage changes the arithmetic. Nobody goes out after one defeat, so a player can lose the opener and still reach the semi finals, and by the last round of group matches the qualification maths can make a match look unlike a normal contest. Sets and games won serve as tie breakers, so margins matter even when a result appears settled. Indoor hard courts reward clean striking and reliable serving, and because these eight have faced each other repeatedly, head to head patterns carry more weight than in an open draw.
The ATP Finals close the men's tour season, bringing together the leading singles players and doubles teams from the ranking race. Two groups of four play round robin matches, the top two in each group reach the semi finals, and every match is best of three sets.
The programme itself is unusual. Each session pairs a doubles match with a singles match, competitors appear on alternating occasions rather than in sequence, and reserves stand by in case somebody has to withdraw. Everything happens in one arena, so nobody travels and the conditions stay identical from the first ball to the last, which puts the focus squarely on execution. The semi finals cross the groups, so a group winner meets a runner up. Service hold percentage and points won behind the second serve explain results better than winner counts.