The Wimbledon champion has struggled in the unusually extreme conditions in NY this year and was again in discomfort during a 6-3 6-4 6-4 victory over Australian John Millman that lasted almost three hours.
Millman, who beat Roger Federer in the previous round, returned after a brief interval but found no joy against Djokovic's serve and the Serb broke the Australian in the penultimate game before serving out the second set.
"I try to leave it all out there - I left it all out there today, but Novak's a champion, " Millman said.
"I was happy to sit there and rest for 10 minutes", he said before breaking Millman in the final game of the set to snatch the set and seize command of the match. It's literally after two games you're soaking, you know. And Roger, he never sweats that much. But we saw him a couple nights ago.
Upon missing a gimme volley - after Millman retrieved a lob with some incredibly scrambling - and then pushing a return wide, Djokovic staggered back into position before tipping water over his head at the changeover.
Djokovic said the tournament needs to address the problem of air-conditioning in an arena which can hold nearly 25,000 people.More news: Maybe Goldman Sachs Isn't So Keen On Crypto, After All
But the heat on the court also led to this brief, odd and delightful moment from Novak Djokovic on Wednesday night. "It feels like sauna".
"Before that I've got Davis Cup which I'm also really jumping out of my skin to team up with the boys there, and anytime you get to play for your country is an awesome opportunity".
Djokovic refused to reveal what the tablets were that he was demanding from his team during the match, while he was also given a nasal spray from the doctor.
He was sweating so profusely that he was leaving moisture on the court, and officials determined that was too risky and allowed the Australian player to change without being penalized.
Millman said: "It was a tricky one coming off".
'I find it a bit odd, they obviously lack a little bit of character'. But that's tennis. That's fine.More news: US Open 2018: Roger Federer Defeated by Australian John Millman
Like Millman did to the 37-year-Federer, Djokovic finally wore down the 29-year-old Aussie. You're dripping. But that's no excuse or anything. "I'd play in a swimming pool if I got to play a quarter-final every week at a Grand Slam".
Djokovic was 1/13 on break points when Millman double-faulted on a 14th opportunity at 4-4, volleying the ball away with his foot in annoyance at giving his opponent what he had been unable to take for himself. Changing a lot of T-shirts, shorts, " said Djokovic, who will face 2014 US Open runner-up Kei Nishikori on Friday.
"From my experience, and I can only talk about myself, this has been definitely the toughest US Open in the last ten years that I have played in in terms of conditions", said the 13-time major victor.
"I'm really excited for that (the Australian Open)". He's truly a great fighter. I think just that makes it a totally different US Open.
The pair have met more than any two men in the Open era and could meet for a fourth time in a final in NY. Add that to No. 20 Naomi Osaka's 6-1, 6-1 win over unseeded Lesia Tsurenko of Ukraine, and Osaka and Nishikori give Japan semifinalists in both men's and women's singles at the same Grand Slam tournament for the first time in tennis history.
The popular Queenslander also said he copped online abuse after his first-round loss to Taro Daniel at Winstom Salem the week before the US Open.More news: Colorado store clearing out Nike products in protest of Kaepernick campaign
"She has won a Masters". I'm also happy for myself, too, being injured a year ago. On two occasions, Millman raced back from the net to deep behind the baseline to make retrievals that shocked Djokovic, who blew the point both times. I always play good here.