Rafael Nadal is in London for the season-ending event, having enjoyed a stellar 2017.
The Spaniard has claimed two Grand Slams and shot to the world No 1 ranking.
However, an age-old knee problem recurred at the Paris Masters last week, which forced him to withdraw at the quarter-final stage.
Nadal has since sought medical help in order to be fit for London.
He will have to play five matches in a week if he is to win the ATP World Tour Finals title for a first time.
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And he confronted the situation when asked if his body can cope with the strain.
“If I believe I cannot do that [play] then I would not be here. I am here to try and win the tournament,” Nadal told Sky Sports.
“You’re on the World Tour Finals, you cannot expect an easy group.
“All the matches are so difficult so you need to be ready from the beginning.”
Nadal has been drawn in Group A alongside Grigor Dimitrov, Dominic Thiem and David Goffin.
Should he get through the group he will play one of Roger Federer, Jack Sock, Alexandr Zverev or Marin Cilic in the semi-finals at the O2.
Nadal has won the China Open and US Open since the summer, as well as reaching the Shanghai Masters final.
And his family doctor Angel Ruiz Cotorro has confirmed he will make a decision on his troublesome knee on Saturday.
“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,” Cotorro said.
“Now he suffers some small discomforts within a pathology that we already know, in a tendon in which he has already had ailments in other occasions, and we will try to do everything possible so that he can be in good condition, because he wants to be, is the only tournament he’s missing.
He added: “If the tendon is holding the loads in a progressive way, you will be in a position to play, if you can not stand it, it is a risk that is not worth running.”