(CNN)The fairytale is over.
John Millman, who memorably beat Roger Federer to advance to the US Open quarterfinals, lost in straight sets to sixth seed Novak Djokovic 6-3 6-4 6-4.
It was a sad comedown for the unheralded Aussie, who put in the shift of a lifetime on Tuesday to oust the five-time winner with a four-set victory.
In humid conditions that seemed to sap both players, the 29-year-old couldn't equal that match's intensity against Djokovic, a two-time US Open champion who's playing in his 12th American slam tournament.
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.
"(The match was) almost three hours -- it's midnight now. Credit to John for putting up a great battle. He's truly a great fighter.
"He's had an amazing tournament, first quarterfinals. To come out here (after Tuesday's win against Federer) and fight for three hours, he deserves a round of applause."
Djokovic consoles Millman after beating him to advance to the semifinals of the 2018 US Open.
Humid conditions
From early on in the match Millman, who had never previously progressed past the third round at Flushing Meadows, looked flustered and out of breath.
After four games of the second set, the Australian approached the umpire to request an outfit change due to excessive sweating, which he said was making the court slippery and dangerous.
He reappeared six-and-a-half minutes later, and later changed his white shirt for a black one.
Deeming it an "equipment out of adjustment" provision, the umpire did not penalize the Australian for the break. A smiling Djokovic admitted that he'd welcomed the mid-set rest while his opponent changed.
The Serb said that the trying conditions had made the match difficult for both players.
"(We were) struggling at the end of the first set," he said. "Just trying to hang in there, trying to find a way to win the match. It's happened a couple of times in this tournament that you survive on the court and thrive with a win."
Millman reacts after losing a point to Djokovic during their quarterfinal match.
Marching on
The win meant that Djokovic advanced to his 11th straight US semifinal -- a streak only bettered by Jimmy Connors. He praised the Arthur Ashe crowd and thanked them for staying up to cheer him on.
"There are so many people that stayed at the stadium and its past midnight. These guys love tennis ... it's a pleasure to come back to New York and play in front of you guys," he said, addressing the fans.
If Djokovic wins in New York, he'll join Pete Sampras on 14 grand slam triumphs, a total only bettered by Roger Federer and Rafael Nadal.
This year's edition has been fairly smooth sailing for Djokovic, who has only dropped two sets en route to the semis and has switched up a gear in his last three matches.
Djokovic will meet Japan's Kei Nishikori in the last four having already swatted aside Hungary's Marton Fucsovics, American Tennys Sandgren, Richard Gasquet of France and the Portuguese Joao Sousa.
Kei Nishikori reacts to defeating Marin Cilic in five sets at the US Open quarterfinals
Japanese players thrive
Nishikori, a finalist in New York in 2014, will feature in his third US Open semifinal after defeating No. 7 seed Marin Cilic in a grueling five-set encounter, 2-6 6-4 7-6 4-6 6-4.
The victory for Nishikori means that for the first time in the open era there will be a Japanese man and woman contesting the semifinals of a grand slam as Naomi Osaka cruised past Ukraine's Lesia Tsurenko 6-1 6-1.
"I hope people were watching us," said Nishikori. "Hopefully many people were cheering in support.
"Naomi's doing well, because she has won a Masters (Indian Wells in March). I think she can win a title now. I feel it's a big chance for her."
Osaka will face home favorite Madison Keys, who defeated Spain's Carla Suarez Navarro 6-4 6-3 on Wednesday. Keys recorded her best result at a grand slam at last year's US Open when she finished runner-up to Sloane Stephens.