Six-time champion Djokovic is back in the Chinese capital for the first time in 11 years and extended his winning run to 31 victories and no defeats on Friday, but only after dropping a set for the first time since the same round in 2013 and again taking a timeout before finishing strongly in a 4-6 7-6 (7-2) 6-2 victory.
Djokovic opened up about an ongoing medical condition he is managing after an early loss in Cincinnati in August, and he appeared to struggle with his breathing early in the second set against home hope Bu.
But he again insisted that this year has been a massive struggle, not due to stamina issues but because of the undisclosed health problem he also suffered at the US Open.
Having lost the opening set on Friday, the Serbian looked in danger when he let an early break in the second go, accepting a time violation as he fought to regain his breath between points.
Djokovic then consulted the trainer, as he had in his first-round win over Nuno Borges, but he dominated the tie-break and Bu was unable to maintain his high level in the decider.
"I don't want to go into details because then it's going to sound like I'm whining about something or looking for excuses," Djokovic told a news conference.
"Truth is that this year has been a big, big struggle for me with few particular issues that I've been having.
"I've been just trying to find different ways to be at least giving myself a decent condition physically and health-wise to be on the court and compete with the player across the net.
"Most of the matches and tournaments this year that I've played have been a real, real struggle for me physically. That's nothing to do with stamina. People mix it up. It's not stamina. It's something else. More related to health."
Djokovic did not elaborate on the problem but said he and his team were still trying to get to the root of it.
"Actually surprisingly something I haven't experienced this year so far, that in the third set I kind of broke through. I felt better. Towards the end of the second and all third set, I felt fresher than I have in the beginning of the match...," he added.
"It's not fun when you're feeling like that on the court because then I'm investing my emotional, mental energy, and physical of course, more on the issues that I'm having rather than thinking about the game and how I can beat my opponent."
If Djokovic is to win another title, he is likely to have to beat US Open champion Alexander Zverev next, with the German taking on Shang Juncheng on Saturday for a place in the quarter-finals.
