Nadal, 31, has a right knee injury that forced him out of the Paris Masters this week.
He pulled out before his quarter-final match and subsequently had scans on the problem, which showed no more damage than there previously was.
It means his chances of playing at the O2 hang by a thread but supporters have taken to his official Facebook page to tell him to stay away.
One wrote: “Rafa dont you worry, you have already had a tremendous year, if you dont have your health, you have nothing, take a break, get your treatment done, and put London out of your mind, we want to see you healthy for 2018, we want to see you haul the Australian Open Trophy, and an 11th RG trophy, taking care of your health is more important than playing in London.”
Another emotional fan wrote: “Your health is the most important thing. We want you in tennis as long as possible. Don’t think about the tournaments. If you feel pain, just don’t play.”
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A fellow Nadal supporter said: “Wish you a full recovery, don’t push yourself. Back when you’re ready, I wish to see you in London but if you’re not ready then see you in 2018. Your heath is above all and more important than trophies,”
While another added: “Well done Rafa get well soon for Australian open if i was u i’d withdraw from the ATP finals too.. Amazing season our champ.”
Roger Federer’s withdrawal from Paris means Nadal has already sealed the world No 1 spot.
The Spaniard has reached the final of the end-of-season championships twice but has never won the tournament. He will now undergo intense treatment this week in a bid to be able to play.
Another fan urged Nadal: “Your health has the most priority now and listen to your body, your knee needs time to heal so let the WTF for what it is, you have reached so much in your career so focus on 2018 for the AO, wish you all the best, take care and God bless.”
While one simply wrote: “Rafa. Just take the rest of the year off and pleaseee forget london.”
The tournament kicks off on Sunday with the final a week later.