The Spaniard has been troubled by appendicitis in recent weeks and is undergoing a course of antibiotics to avoid having surgery so he can compete at the season-ending tournament.
Following his second-round defeat at the Shanghai Masters to Feliciano Lopez last week, the 28-year-old hoped he would be able to manage the condition, but now admits his World Tour Finals presence is far from guaranteed.
"I am sad because I didn't have the chance to compete [in Shanghai]," he told Sky Sports News. "I was in a big process of antibiotics for a week-and-a-half and the body goes down, you need time to recover to get your top physical performance.
"I have had weeks of injury so it was not an easy season for me at the end, I have to do the surgery but I have to find the right moment.
"I need to see how things go and how my body accepts the antibiotics, so after all the things that have happened over the last few weeks - let's see.
"I will try to do my best [to be in London] but I cannot say. I do not know a hundred per cent."