Rafael Nadal pulled out of the Paris Masters last week with a right knee injury, but is top seed here, with perennial rival Roger Federer second seed at the O2 Arena.
'If I didn't believe I could be ready for Monday I wouldn't be here,' Nadal said at the pre-tournament media conference.
Rafael Nadal is adamant he will be fit to play in the season-ending Nitto ATP Finals
'I don't have to make a decision. I'm going to play. That's my feeling today.
'I had problems in Paris a week ago. I go day by day, how I'm feeling.
'I cannot predict what will happen, but my feeling now is I'm going to play and try my best.'
Nadal faces David Goffin of Belgium on Monday evening in the last of the first round of matches. Two groups of four players play in a round-robin format before semi-finals and a final.
Nadal insisted he had made no schedule demands on the organisers which would give him additional time to recuperate from his injury.
The Spaniard was forced to pull out of last week's Paris Masters with a knee injury
The world No 1 opens his campaign against Belgium's David Goffin on Monday night
'I didn't have any request. I didn't ask anything,' he said.
With last year's winner Andy Murray and runner-up Novak Djokovic absent through injury, there will be a new finals champion for the first time since 2011, when Federer won for a sixth time.
Nadal is guaranteed to end the year as world No 1, irrespective of his performance at the finals, which begin on Sunday and end on November 19.
He has qualified for finals previously 12 times, withdrawing on five occasions, including 12 months ago.
And Nadal, who won the French and US Opens in 2017 to take his grand slam tally to 16, has never won the year-ending event.
'It's always in my mind, during 13 years that I'm qualified,' the 31-year-old said.
Nadal won two Grand Slams in 2017 and is set to end the year as world No 1 once again
'Being here is an important thing, but for me the most important thing is the only thing that happened to me this year. Winning tournaments, competing every week, being healthy until Paris.
'I'm No 1 here, great. It's a very important achievement, especially at my age.
'It's something important for me, but that's all. It's going to be a nice moment when I receive Sunday the award, a year ago it was never in my mind to achieve that again.
'For me the most important thing is to finish the season well.'
On the prospect of playing Federer in the final, Nadal said: 'It would be great to finish the year playing against him again.'
Roger Federer and Nadal headed the party at the launch of the ATP Finals in London
Nadal faces the prospect of meeting second seed Federer in the final in London next Sunday
World No 7 Stan Wawrinka is also absent due to injury, so there are some new faces in the competition.
Federer opens on Sunday afternoon against Jack Sock of the United States, while third seed Alexander Zverev of Germany plays Marin Cilic of Croatia.
Austria's Dominic Thiem plays Grigor Dimitrov on Monday.Read more at dailymail.co.uk