The injury-plagued Spaniard, who has only recently returned from a wrist problem, is on antibiotics and hoping to delay surgery for appendicitis until the end of the season.
However, with the eight-man London showpiece looming, the world number three said he was not certain he will be there.
"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," the 14-times grand slam champion told Sky Sports.
"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 percent."
Nadal, along with Novak Djokovic, Roger Federer and Stanislas Wawrinka, has already qualified for the tournament with U.S. Open champion Marin Cilic also virtually guaranteed a debut appearance.
The likes of Andy Murray, Tomas Berdych, David Ferrer and Kei Nishikori are scrambling for the remaining three spots.
(Reporting by Martyn Herman; editing by Toby Davis)
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