The Serbian advanced to the last 16 at Roland Garros on Saturday thanks to a 6-2 6-3 6-3 win over Aljaz Bedene.
Still searching for his maiden grand-slam title in Paris, the 11-time major winner's prospects improved thanks to Nadal's withdrawal due to a wrist injury.
But Djokovic said it was sad to see the Spaniard exit the year's second major because of injury.
"It's definitely not very commonly seen to have [Roger] Federer and Nadal not in the second week of the grand slam and top players getting injured," he said.
"Of course it's not something you like to see as a spectator and tennis fan and as a tournament organiser, because the top players are very important for the tournament, but it happens."
Djokovic added: "If he retired from the tournament, then something really is serious, because he's one of the greatest competitors that the game has ever known.
"It's sad to not have him in the tournament. But again, we keep on going, and I'm still in and I'm just hoping it's going to go well for me."
Eyeing his first French Open title, Djokovic will also be aiming for his maiden Olympic gold at the Rio Games later this year.
Despite concerns over the Zika virus in Brazil, the 29-year-old expects the Games to go ahead as planned.
"For the moment I'm planning to play the Olympics. Nothing is changing, but let's see," Djokovic said.
"Let's see what happens because to even think to cancel the Olympic Games is unthinkable, honestly.
"I mean, many athletes and people already planned in advance and so many people already planned their trips and accommodation in Rio.
"Of course we have to have common sense that health is most important for anybody that is there.
"But we should not only think about people coming in Rio. How about those people living there, you know? Not talking about them too much. So I think we have to look from different perspectives in order to make a right conclusion."