The Spaniard has been forced to take antibiotics to delay surgery on his appendix, and he is concerned by the possibility of a negative response on court.
"It's going to be one of the most dangerous first rounds in a 500 tournament that I've played in a long time," said the 28-year-old after practicing on Sunday.
"I am not 100% sure what is going to happen tomorrow. I haven't practiced well enough or enough times. My body feels more tired than usual when I'm playing.
"I'm a little bit scared about how my body is going to react, some of my injuries have been after antibiotic treatments. Hopefully that won't be the case this time."
Nadal has not won a title since the French Open in May.