The 38-year-old Serbian also appeared unwell on several occasions, and fell to the floor as he lost the second set after a lengthy rally.
Djokovic called for a medical timeout for his ankle after breaking to take a 3-1 lead over Munar in the first set, and once again, required treatment after consolidating his advantage.
While the world No. 5 was still able to wrap up the first set, he struggled through the second before being tended to by a trainer, who assisted him to his chair and checked his blood pressure.
But when Djokovic returned to court, he had a new lease of life, opening the third set by breaking Munar’s serve. He did so again to take a 5-2 lead before wrapping up a gruelling win in two hours and 41 minutes.
Next up for Djokovic, who is looking to win a fifth Shanghai Masters, is an encounter against world No. 44 Zizou Bergs in the quarter-finals.
Rune calls for ATP to introduce heat rule
Elsewhere, Holger Rune progressed to the quarter-finals by beating Giovanni Mpetshi Perricard 6-4 6-7 (7) 6-3.
After the match, the 10th seed called for the ATP Tour to introduce a heat rule.
“Today was better, as you say, today was less hot. But again, it was less hot, but if it was a more normal temperature today would be very hot. I think it was like 31 degrees and very humid. But compared to the other days it was pretty brutal.
“So I think there should be some kind of rule. We can handle a certain amount of heat because, you know, we are fit, we’re strong, we’re mentally strong as well, but there’s always a limit. I think it’s also important to take care of your health. We need to survive.
“I think some kind of rule, you know, not that we should stop if it’s just more than this, but like the Grand Slam, you know, they have a feeling on when it’s a little bit too much, and we wait a little bit to see if it drops.”
