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.
Millman, though, also leaves Flushing Meadows a victor and with renewed belief that he can challenge for major spoils following his stirring run to the second week of a grand slam for the first time.
Djokovic moved a step closer to a third championship at Flushing Meadows and 14th Grand Slam title overall by eliminating Millman 6-3, 6-4, 6-4 in the quarterfinals.
"I personally have never sweat as much as I have here", Djokovic said early on Thursday morning after completing his 6-3, 6-4, 6-4 victory. "Everything gets slower as you try to hit winners", Federer said.
Djokovic and Nishikori will meet in the Arthur Ashe Stadium after Nadal and Del Potro clash this evening.
"Well, it definitely means a lot for me, and I always thought if I were to win a Grand Slam, the first one I'd want to win is the U.S. Open, because I have grown up here and, like, then my grandparents can come and watch", she said.
"I guess I felt I couldn't get air", Federer said with a shrug after his fourth-round loss.
"I just have the chance to play the greatest in this sport, and it's awesome for me".
"I was, alongside many other people, anticipating the match against Federer", Djokovic said.
This night was cooler, as the temperature dipped into the 70s, but the humidity was above 80 percent, so with Millman drenched, he sought permission for a wardrobe change at 2-all in the second set.
But Millman asked Djokovic if he'd be OK with a delay after the fourth game.
Djokovic, grateful for a bit of a breather, sat grinning in his chair with his shirt off, much to the crowd's amusement.
"We were both struggling and trying to hang in there". "I'm not normally someone who changes full clothes, full kit".
The Serb was given a time violation warning and double faulted to offer Millman a break point opportunity before losing his first serve after running down the shot clock a second time.
"I think that this tournament needs to address this", Djokovic said.
Meanwhile, Australian Open champion Caroline Wozniacki called out supporters in the stands at the Miami Open earlier this year for abuse suffered by both her and her supporters in her player box. If the stadium reverberated under the chants of "Come on, Roger!" two days back, cries of "Aussie!"
That policy also allows for a 10-minute break between the second and third sets of women's singles semifinal matches on Thursday night.
Nishikori also needed to labour through five sets to beat Marin Cilic and given the effects of fatigue backing Djokovic to win this one in straight sets at [1.99] looks a decent price. "He's a fighter. He fights for every point and is never going to hand you the win".