Brutally hot and humid conditions have taken their toll on a number of players in China this week and, after appearing to vomit during a long previous match, Djokovic was in trouble again during a 6-3 5-7 6-2 win over Spaniard Munar.
After losing a long second set on a gruelling point, Djokovic lay sprawled out behind the baseline before eventually being helped back to his chair by a trainer.
Having already been treated for an apparent left Achilles issue, it appeared Djokovic might not be able to continue, but, after having his vitals checked, the fourth seed roused himself to make it through to the quarter-finals.
In doing so, the 38-year-old, who declined to do the customary on-court interview, became the oldest man to reach the last eight at an ATP Masters 1000 event.
With Carlos Alcaraz absent and Jannik Sinner succumbing to the conditions in his third-round match, this appears a golden chance for Djokovic to win a first Masters title since 2023.
But the Serbian was in trouble after only four games, shaking his head and grimacing after sustaining a problem in his lower left leg, which already had strapping on the hamstring area.
Djokovic was already a break ahead, though, and, while he did not move comfortably after receiving treatment, he held onto his advantage to take the opening set.
He kept Munar at bay in the second set as well, but Djokovic was breathing heavily and frequently doubling over after long points.
Losing the set appeared like it could be a decisive blow but, as he has so many times during his career, Djokovic climbed off the canvas and played his best tennis of the match to set up a last-eight clash with Belgian Zizou Bergs.
Tenth seed Holger Rune also survived a battle, defeating giant Frenchman Giovanni Mpetshi Perricard 6-4 6-7 (7-9) 6-3.
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