The hard-won victory keeps Djokovic on track as the favourite to take a record-extending fifth title in the Chinese financial hub, with other top seeds already withdrawn or knocked out.
The 38-year-old almost looked unable to continue when the match went to a decider, but he conjured up the energy to break his decade-younger opponent twice in the third set.
“Thank you, thank you everyone, I love you!” he shouted in Chinese to the adoring crowd before leaving court almost immediately after play finished.
The 24-time Grand Slam champion had a good start despite the 84 percent humidity, breaking Munar in the fourth game.
Djokovic’s problems began shortly afterwards when his left ankle slid out behind him as he rushed towards the net, and he had to take a medical timeout.
He grimaced in pain as he was attended to by an on-court physio, but managed to resume to hold the fifth game.
Djokovic received treatment again in the second set, and looked increasingly fatigued, crouching over frequently in between rallies and throwing up on court.
Chinese fans waving Serbian flags roared in support, but 41st-ranked Munar ultimately broke in the 12th game of the second set after an unforced error from his opponent.
An agonised Djokovic dropped to the court and remained splayed out as a medic rushed over.
But he picked himself up for a compelling third-set revival, breaking in the first and seventh games and sending the stadium into a frenzy. (AFP)
