The world number one's participation had been thrown into doubt because of an ankle injury that he sustained while on Davis Cup duty with Serbia a fortnight ago.
However, he shook off the complaint to turn out at the event and went on to end Nadal's eight-year winning streak in the process.
"I cannot ask for a better start to my clay season," said Djokovic. "I did not know if I was going to be a part of the tournament this year. I made a decision to play and, right now, it's the best decision of my life."
Djokovic had never won the tournament before today.