Millman dragged Djokovic into long, searching rallies and four-time US Open champion John McEnroe said the Australian's level of play was "similar" to the standard at which he played in his memorable fourth-round win.
Djokovic, who was won all 11 of the quarter-finals he has played here, beat Millman 6-3, 6-4, 6-4 in two hours and 48 minutes on another sweltering night in Arthur Ashe Stadium.
Djokovic said after the match that he'd been "very tested", and paid full praise to his opponent after the 2 hour, 49 minute match. This will be the 11th time in 12 years he's appeared in a US Open semifinal. This is her first career Grand Slam semifinal in what's been a year of firsts for her so far.
"But every time you have these moments and these experiences where you play these types of players and you're playing and you think, "Hey, I can match it with these guys".
This night was cooler, as the temperature dipped into the 70s, but the humidity was above 80 per cent, so with Millman drenched, he sought permission for a wardrobe change at 2-all in the second set.
"But definitely it was not easy conditions to play in". He said he had never sweated as much and echoed Federer's comments about a lack of air in the stadium, saying it was a problem the tournament needed to address.
"I didn't even know the rule", said Millman, whose request to leave briefly was permitted based on something called the "Equipment Out of Adjustment" provision in the International Tennis Federation guidelines, because his sweat was making the court slippery. "It feels like sauna".
US Open 2018: Djokovic marches past Millman into semi-final
That came during an unusual stoppage after four games of the second set when Millman requested to go off court to change his whole kit - organisers clarified later that the break was permitted because the Australian was sweating so much it was making the court risky.
"I was struggling, he was struggling, we were all sweating, changing clothes".
Djokovic, grateful for a bit of a breather, sat grinning in his chair with his shirt off, much to the crowd's amusement.
"He doesn't miss a lot himself, so he just runs left, right, gets a lot of balls".
I always think it is a good thing when the victor of any given sporting contest has a moan after the match. I hope you die slowly, but f***ing painfully.You have deserved it!
The Serb, twice champion at Flushing Meadows, has now won all 11 of his quarter-final matches in NY.
The No. 7 seed faces No. 21 Kei Nishikori on Wednesday, four years after beating the Japanese player for his first Grand Slam title. "Sorry", Osaka said after her quarterfinal victory.
Either side of the unusual interlude, Millman showed enormous character to save seven break points, having successfully staved off five out of six in the first set.