The interruption, however, did little to put Djokovic off his game as he continued to threaten the all-action Aussie's serve, eventually breaking through in game nine at the third attempt as he took a two-set lead. "It feels like sauna".
Keys won all 10 of her service games, saving the only two break points she faced.
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'll have some probably now", Millman said before departing NY on Thursday [local time].
It's been a rather incredible journey to the quarter-finals for 29 year-old Brisbanian, whose run to the final 8 included victories over 2nd seed Roger Federer and 14th seed Fabio Fognini.
Djokovic, who next faces Kei Nishikori in a repeat of the 2014 semi-final which he lost, said: "I was, I think alongside many other people, anticipating the match against Federer, and it was a surprise to see Millman win the match". One came in the last game as she served for the victory, but she erased it with a forehand victor, part of a 22-10 edge in that category.
"I'd just like them [the trolls] to be named and shamed a bit more".
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"I don't think that's a normal occurrence", John Isner said on Tuesday after his quarterfinal loss to Juan Martin del Potro.
Millman, whose forehand can be shaky, couldn't sustain it like he did against Federer, when he posted his first-ever victory against a top-10 player and extended Federer's 10-year Flushing drought.
Over the past few years, Millman has learnt to deal with the gutless bullying from strangers, but says it can be distressing for other players on the tour. He's truly a great fighter.
The Serb easily progressed into the final four of the NY tournament, beating Australian rookie Millman in straight sets on Thursday evening. "I didn't want to show this kind of game in front of this big crowd, but unfortunately I'm just not able to play now". Just an all-around player. "He made me run and he made me earn my match today". 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.
"It's great to see", said Nishikori, who is into his third major semifinal - all in NY - but is still in search of his initial Slam trophy.
"I've gotten a lot better managing my emotions once it gets to this part and knowing that everything is going to be probably more amped up", she said. "I am also happy for myself".
He could not find a way back into that set but he made Djokovic work very hard for it, and the Serbian's frustration levels increased during the second set as chances came and went.