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The 22-time grand slam champion has been struggling with a left hip issue that contributed to his second-round exit at the Australian Open in January.
Nadal pulled out of hard-court contests at the Indian Wells Open and Miami Open as he continued to recover from the problem.
Speaking two weeks ago, Monte Carlo Masters director David Massey said Nadal was the first player to register for this year's Masters 1000 event, which begins on April 8.
The tournament's official Twitter account more recently suggested Nadal will "definitely" take part.
However, Nadal – who has slipped out of the world's top 10 men's tennis rankings for the first time in almost 18 years – denied that is the case.
"I don't know who gets this information," he said.
"Obviously if it were true, I would confirm it, but unfortunately I can't. I'm following my course and I don't know when I'll play again, that's the truth.
"I'm in a phase of increasing work. If I knew when I was going to return, I would say so, but I don't know.
"I can't confirm that I will play in Monte Carlo. Things are seen day by day. I prefer to say things when I really know them."
The Monte Carlo Masters kicks off the clay-court season ahead of events in Madrid and Rome, with the French Open – Nadal's favourite event – now just two months away.
Nadal has won the Monte Carlo Masters a record 11 times, though his most recent triumph came in 2018 with victory over Kei Nishikori in the final.