The Serbian superstar downed Millman 6-3, 6-4, 6-4 in a drama-charged war of attrition to reach his 11th semi-final in NY.
They'll likely face the toughest of conditions as they open action on Arthur Ashe Stadium at noon (1600 GMT).
"I asked the chair umpire whether they are using some form of ventilation or air conditioning down at the court-level side, and then he says that he's not aware of it, that, you know, only what comes through the hallway type of thing", Djokovic said. Novak won just two points more than Roger and he had to dig deep in his service games to emerge as a victor before the deciding set, saving 19 out of 23 break points and stealing Federer's serve six times from 13 opportunities.
Conjure an image of Roger Federer in action - his hair flopping perfectly, his feet seemingly never touching the ground.
He is now even money to be this year's US Open victor and I'm not remotely tempted to press the Cash Out button.
"I was, alongside many other people, anticipating the match against Federer", Djokovic said.
"Whether it's night or day, we just don't have air down there", he said. "It feels like sauna".
American player Nicole Gibbs told CNN a year ago that she'd first started receiving online abuse while playing college tennis. He also received pills and a bottle from his team.
Novak Djokovic put aside all of it, from his opponent's unheard-of, middle-of-a-set chance to change out of sweat-soaked clothes and shoes to consecutive time violations because Djokovic let the serve clock expire to the 16 break points he wasted.
Millman said: "It was a tricky one coming off".
I always think it is a good thing when the victor of any given sporting contest has a moan after the match. "We just felt we needed to address that a little bit this year". But that's tennis. That's fine.
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. Millman broke Djokovic's serve just once, whereas Djokovic broke Millman's serve four times. For men's singles matches, the break would come between the third and fourth sets.
Cilic has made two grand slam finals since but Nishikori has never been back and will now take on either twice U.S. Open champion Novak Djokovic or unseeded Australian John Millman for a spot in Sunday's final. He was missing a lot. "But the challenge of playing him on hard of course is even higher for me personally than playing against him on clay". He's truly a great fighter. I think he has had probably the tournament of his life and a match of his life against Roger.
"Unfortunately during this tournament I had many issues with my health, and today was not my day obviously".
After Naomi Osaka reached the women's semi-finals earlier, Nishikori's win means there is a Japanese man and woman in the last four of a Grand Slam for the first time in the Open Era.
Later Wednesday, Osaka was joined in the record books by compatriot Kei Nishikori who reached his third semi-final at the US Open. She did both in the same year, with her 2010 French Open title helping propel her to No. 7 that year.