As his opponent limped increasingly heavily between points, Del Potro ruthlessly fired in winners, his backhand proving nearly as potent as his famously magnificent forehand.
Canada's Leylah Fernandez's U.S. Open run ended in the girls' quarter-finals.
"What. A. Finish", the US Open tweeted after the Serbian won, adding that he cruised to the final in Flushing Meadows after defeating Nishikori 6-3, 6-4, 6-2. Djokovic, another player whose recent injury cost him time at the top of the tennis pyramid, won Wimbledon this year and is rounding into his elite form again.
This night was cooler, as the temperature dipped into the 20s, 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.
Rafael Nadal of Spain reacts during his men´s singles semi-final match against Juan Martin del Potro of Argentina on Day Twelve of the 2018 US Open at the USTA Billie Jean King National Tennis Center on September 7, 2018.
As Del Potro concentrates on his opportunity to win a second Grand Slam, the outgoing champion will instead have to ponder his future and wonder what might have been, were it not for yet more injury woe. I am playing well. "Help me make sure she sees it!"
"That shows the quality that he has", Djokovic said.
"He's a gentle giant", said Djokovic of his giant Argentine opponent.
That's what I try to do tomorrow, with the ice bath, then massage, then came for hit the ball a little bit also.
Nishikori said he was happy to have made it to the semi-finals.
Camera Icon Novak Djokovic returns a shot during his straight sets
It was Nadal's second retirement from a Slam this year - both on hard courts - after Australia.
Nadal said he didn't know what kind of effects might have been lingering from his quarterfinal victory over Dominic Thiem, which lasted five sets and almost five hours.
"This year have been a fantastic year until this moment", said Nadal. He said the discomfort surfaced in the first set, with him serving at 2-2, 15-0, and he told his coaches and supporters in his box about it.
Yeah, I waited as much as I can. "You can imagine, it was very hard to say goodbye". But, you know, hopefully I can get one better in few days'.
'It's definitely one of my favourite tournaments to play because of the conditions and because of the fact that I've played so well in each year that I keep on coming back to the US Open.
Nadal already had the admiration of tennis fans for his relentless approach and unyielding competitive spirit.
Del Potro, a man far too familiar with his body letting him down, is probably the last person who would have wanted to progress in such a manner but surely no player deserves a kind break more. He and Nadal have developed a respect for each other.
When Nadal missed a forehand wide, then put a backhand into the net, the opening set belonged to del Potro, who shook his fist and yelled, "Vamos!"
"All my career everybody say that because of my style, I will have a short career".
Another hurricane is barreling toward Hawaii
The NHC said Olivia will weaken further over the weekend, and possibly downgrade to a tropical storm by Tuesday. Hawaii residents are on edge again as the latest storm threat shows Hurricane Olivia headed for the Islands.