The second seed will get his campaign for a sixth Madrid title under way with a first-time meeting with Canada's fast-rising teenager Felix Augur-Aliassime, in the second round.
The Spaniard, who turns 33 next month during Roland Garros, said that while he might not be at 100 percent for the match, he will go in fighting.
“I've been going through a virus. The positive side is: I've been able to train at least an hour-and-a-half (per session),” Nadal said.
“I hope that tomorrow is going to be a bit better, I'm going to improve.
“I'm a bit better than yesterday and yesterday was better than the day before.”
But the 17-time Grand Slam winner warned: “It's true that when you have this virus and stomach viruses, the body's a bit weaker.
“There is a bit more risk of injuries. But of course, this is a very important tournament for me and I hope to be ready.
“Hopefully the virus is not going to affect my performance or how I should play tomorrow.”
Nadal said that while he would have loved to have arrived in the capital with titles in his pocket, the lack of them is hardly a crisis; he lost in semi-finals in Monte Carlo and Barcelona, two events where he was defending titles.
“I will try to do my best, look at the positive things and go step by step,” he said.
“Barcelona (where he lost to Dominic Thiem) was a positive improvement for me. I'm feeling better now than then – I'm on the right path.
“I'm building up my game with more confidence than I had in Monte Carlo. I'm enjoying training and I enjoy playing each day.
“This is a very important step for me.”