The 38-year-old Spaniard, dubbed the "King of Clay", has dropped to 161st in the world rankings after struggling for two years with injuries and suffered one of his worst losses against Djokovic in a 6-1 6-4 defeat.
"I cannot spend every day thinking about whether it is the last or not, because otherwise it is impossible," he said.
Nadal is still in the doubles with Carlos Alcaraz.
"I have been suffering a lot of injuries the last two years so if I feel that I am not competitive enough, or physically if I am not enough ready to keep going, I will stop and I will let you know," he said.
The record 14-times French Open champion had hesitated to drop the singles to focus on the men's doubles competition in which he plays with world number three Alcaraz.
"I have the doubles left. It is not time to get down," he said. "When I finish here I will make the decisions I have to make here."
Djokovic, 37, was full of admiration for his long-time rival and said he did not know whether he would play Nadal again, but that in any case both of them would need to be selective in their tournament choices going forward.
"I think we both want to play in the Grand Slams and the biggest tournaments," he said.
"I just hope for the sake of our rivalry and the sport in general that we'll get to face each other once or maybe a few times on different surfaces and in different parts of the world because I feel like it can only benefit the sport."
