"I have done everything possible to reach the end of the season in good condition, both in Paris and London, and I really wanted to play", Nadal said.
On his page on "Twitter" Nadal reported that the early end of the season he was inclined to numerous health problems.
Djokovic will be the ATP's year-ending No. 1 for the fifth time, pulling even with Roger Federer and Jimmy Connors for the second most since the computer rankings began in 1973.
"This way I hope to be in full condition for next season".
Novak Djokovic reclaimed the world No.1 ranking on Monday after a two-year absence and is assured of finishing the year there.
Abdominal problems had seen Nadal pull out of the Paris Masters last week and while they continued to hinder him, the Spaniard also revealed he needed an operation to remove a "free body" from his ankle.
"Reflecting on what I've been through in the a year ago, it's quite a phenomenal achievement", Djokovic said.
Unfortunately, Rafael Nadal and Juan Martin Del Potro have withdrawn from the tournament due to their various injuries, giving Japan's Kei Nishikori and the big-serving American John Isner the two remaining spots in the glittering field.
Cape Town - It's a case intriguingly of what he can achieve, what he can't achieve and what he will achieve when top South African tennis player Kevin Anderson competes in his first ATP Finals next week. "I couldn't be happier to finish the season like this". Djokovic, Federer, Zverev, Anderson, Cilic, Thiem, Nishikori and Isner are the eight aspirants to win the Finals.
Only a semi-final defeat at the French Open prevented him from becoming just the third man to capture a calendar Grand Slam.
Armenian Karen Khachanov, a rising star in the world of Tennis from Russian Federation, defeated incoming world champion Novak Djokovic to win his first Masters 1000 title at the Paris Masters, reported the BBC.