Andy Murray may have entered the record books after surging to world number one at the Paris Masters but his achievement will count for little if the Briton cannot hold onto his place at the summit of the ATP standings and end 2016 as the planet's premier player. The 29-year-old holds a wafer-thin advantage over the 12-time grand slam champion Novak Djokovic heading into the season-ending World Tour Finals at The O2 Arena and must be at his very best if he is to conclude the campaign top of the rankings.
Murray will defend his status for the first time in London, at an event in which he has regularly struggled to make an impression. The Wimbledon and Olympic singles champion has never reached the final in seven appearances in Greenwich, and on four occasions has failed to progress from the round robin phase. If the British number one is to improve on his record at the event he must do it the hard way, with Stanislas Wawrinka, Kei Nishikori and Marin Cilic drawn in his group.
Djokovic meanwhile has a far simpler task if he is to draw level with Roger Federer by claiming a record-equalling sixth title at the ATP Finals and end the year as world number one for a fifth time. The Serbian will face injury-doubt Milos Raonic and debutants Gael Monfils and Dominic Thiem in his group, knowing he has never lost to the trio in 23 matches. Two victories in the pool stage and passage into the semi-final will ensure that Djokovic's destiny is in his own hands as he pursues Murray in the English capital.
The Dunblane-born player leads Djokovic by 405 points heading to the ATP's flagship event, but will lose the 275 points he gained from helping Great Britain win the 2015 Davis Cup. This effectively reduces Murray's advantage to just 130 points, meaning every match this week will be critical in determining who ends the season as the number one player.
Both Murray and Djokovic have had the points they won at last year's event already subtracted from their total, essentially meaning neither have anything to defend in London. The ATP awards 200 points for each round robin win, 400 for winning the semi-final and an additional 500 for claiming the title. An unbeaten champion walks away with 1,500 points and in the case of Murray or Djokovic is all-but guaranteed to finish the year as the king of men's tennis.
Put simply, Murray must match Djokovic's performance to ensure he successfully defends his ranking – or beat the Australian and French Open winner in the final should both players meet [assuming Djokovic has qualified with two round robin wins]. Murray can in fact stay as world number one if he reaches the final with a 100% record, and Djokovic only claims the title having claimed one round robin victory.
Only when either player is eliminated early in the competition, as has happened with Novak in each of his last two Masters Series 1000 appearances, do things get complicated. Two group wins is traditionally enough to progress, while one or less is generally not enough unless there is an injury withdrawal. If one or both of Murray or Djokovic is eliminated in the preliminary round, then the opposing player need only have recorded more round robin wins to guarantee they claim world number one.