"It is not about London, it is about the longer term" he added.
This is his fourth time to be crowned world No 1 at the end of the season, after 2008, 2010 and 2013.
The Spaniard, who had his right knee strapped during his third-round win on Thursday, said he wanted to try and be ready to face Filip Krajinovic in the last eight but that "it wasn't possible to play". It's something that means a lot to me.
World number one Rafael Nadal scored his 67th victory of the season and 345th at Masters 1000 level overall after a hard-fought battle against Pablo Cuevas, prevailing by 6-3 6-7 6-3 in two hours and 20 minutes.
Nadal's withdrawal means his record of having never won the Paris Masters goes on.
United States to issue import duties on Canadian softwood lumber
The ministers said Ottawa will turn to litigation if required to defend the industry and expects to prevail as it has in the past. Canadian unions and lumber companies fear the issue will eventually cause layoffs once prices and demand fall.
The Spaniard withdrew from the Paris Masters on Friday, and Roger Federer, Novak Djokovic and Andy Murray - all missing from the tournament - have also been sidelined by injury during the season. He didn't say whether he will be able to play at the ATP finals in London, which starts No. 12.
"Of course it's a very tough decision for me, especially here in Paris, which has been the most important city in my career without a doubt", said Nadal, a 10-time French Open champion.
Conversation soon went onto the demands of the year-long ATP schedule, the increasing number of cases of high-profile players taking an extended break from the tour and whether changes need to be made to the scheduling of tournaments in the ATP calendar.
The last spot is now held by Spain's Pablo Carreno Busta but Juan Martin Del Potro, Roberto Bautista Agut, John Isner and Sock still have a chance to qualify depending on results in Paris.
"I have to pull out". The way that I am today I do not see myself playing three more matches.