Nadal announced Thursday that he will step back from tennis after the Davis Cup finals in November, ending his 22 Grand Slam-winning career.
He clashed 60 times with Djokovic, who edged their rivalry 31-29.
"He remains the greatest rival that I ever had. He has impacted me a lot as a player, my development, he has inspired a lot of people around the world," said Djokovic, speaking after he beat 19-year-old Jakub Mensik in three sets to progress to the Shanghai Masters semi-finals.
The 24-time Grand Slam champion said that the news had come as a shock despite the Spaniard's struggles with injury.
The Serb is now the last man standing from tennis's golden era, with Roger Federer retiring in 2022, and Andy Murray calling it quits this year too.
"I'm playing, you know, still keep going... but part of me left with them, that's for sure," mused Djokovic.
"The era of the four musketeers, so to say, the four of us and all the rivalries we had was incredible," he said.
"I'm a bit overwhelmed, to be honest, but, you know, I still have the desire to play.
"Fighting against a 19-year-old for two and a half hours on the court is something that still drives me... and I try to get the best out of myself."
