The world number two was set to face Feliciano Lopez at 2am this morning in his first-round clash but he has since pulled out through injury.
It is the fifth consecutive tournament Nadal has been forced to withdraw from and he has now cast doubt on returning in time for the Indian Wells Masters next week.
The Spaniard had planned to play in California in a bid to catch Roger Federer at the top of the world rankings.
The Swiss star had leapfrogged to the summit with success at the Rotterdam Open last week.
However, he may now be able to extend the gap at the top if Nadal does not return from injury quickly – and he has serious doubts.
“I went to the hospital this morning to check it and to do an MRI [scan] and I still have some liquid there,” Nadal said.
“I need to keep doing tests to know exactly what is going on.
“The spot is really similar to Australia so we need to be really sure what is going on in the next couple of days.
“Then we will work hard to recover as quickly as possible.
“It is impossible for me to say something realistic about if I am going go be at Indian Wells or not.
“My goal is to be there and I am going to work to try and be there but I can’t say yes or no know.
“I hope the tests will be as positive as possible and then lets see.”