Rafael Nadal desperately wants to add the ATP Finals title to his vast collection of silverware but the world number one will only play if his knee responds to treatment.
Nadal has twice been runner-up in the last event of the year - in 2010 and 2013 - and with Novak Djokovic and Andy Murray both injured, 2017 represents a great chance to secure the title for the first time.
The Spaniard ensured he would finish the year at the top of the world rankings during the Paris Masters, but he withdrew ahead of his quarter-final with Filip Krajinovic due to injury.
That put his place in the end-of-season tournament in London in doubt, and his doctor has stressed that, if he is not fully fit, it would not be worth taking the risk of playing.
"You always have to be optimistic, but also realistic, you have to see that we are at the end of the year, after an extraordinary season," doctor Angel Ruiz Cotorro told El Larguero.
"Now he [Nadal] suffers some small discomforts within a problem that we already know [about], in a tendon in which he has already had ailments on other occasions.
"We will try to do everything possible so that he can be in good condition, because he wants to be, it is the only tournament he's missing.
"If the tendon is holding the loads in a progressive way, you will be in a position to play, if it cannot, it is a risk that is not worth taking."