Novak Djokovic and Rafael Nadal are the most serious challengers to end the year as the world No 1.
Australian Open and Wimbledon winner Djokovic is 640 ATP points ahead of Nadal in the standings.
However, the Serbian must defend 2,600 points over the coming months, meaning one slip-up could hand his Spanish rival the top spot.
Nadal has got three events remaining this term – the Shanghai Masters, Paris Masters and ATP Finals – and he has a free shot at each of them having sat out of the end of 2018 through injury.
Realistically, Djokovic will have to better last year’s performances and the entrance into next week’s Japan Open should help.
The 16-time Grand Slam winner is the heavy favourite to win the Tokyo title and add 500 points to his tally heading into Shanghai.
That means Djokovic could be 1,140 ahead of Nadal and defending the title he won last year would keep him ahead by at least 500 points.
The problem could arise in Paris, where Djokovic finished runner-up last term.
A title in the French capital could add a further 400 points, while bettering last year’s second-place finish at the ATP Finals would strengthen his case by another 500.
So if Djokovic wins every remaining event he would be 2,040 points ahead of Nadal – minus the total Nadal accumulates.
The 32-year-old came out this week and told reporters in Belgrade he want to secure the year-end world No 1 once again.
Meanwhile, Federer has a very slim chance of leapfrogging both Djokovic and Nadal.
The Swiss star will likely need his rivals to lose in the first-round stage of all the remaining tournaments.
ATP rankings
Novak Djokovic, 9,865 (defending 2,600)
Rafael Nadal, 9,225 (defending 0)
Roger Federer, 7,130 (defending 1,620)
