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July's Wimbledon champion had been drawn to face Federer with a semifinal berth at stake.
Despite having never won a Grand Slam, just 11 career tournaments, Nishikori is in the same league as Grand Slam kings Roger Federer and Rafa Nadal when it comes to pitching products.
'We had some matches where I won in straight sets, but we had a lot of matches where it went the distance where a couple of points really decided the victor.
This night was cooler, as the temperature dipped, but the humidity was above 80 percent, so with Millman drenched, he sought permission for a wardrobe change at 2-2 in the second set. The chair umpire was even seen on the phone during that time. "I tried to swing a little more, play aggressive and start working better from 4-2 down", he added. We were all sweating.
Kei Nishikori is headed to the US Open semifinal following a dramatic upset victory over Marin Cilic on Wednesday. "I was freaking out inside, just like my whole body was shaking, so I'm really glad I was able to play well".
Upon missing a gimme volley - after Millman retrieved a lob with some incredibly scrambling - and then pushing a return wide, Djokovic staggered back into position before tipping water over his head at the changeover.
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.
That made it all the more surprising that the 28-year-old allowed his advantage to slip, missing a break point that would have given him a 5-1 lead and then being pegged back to 4-4. After missing several open shots, Djokovic doubled over and staggered around the court as his camp - coaches, trainer, and wife Jelena - looked on worriedly.
But Djokovic broke back in the match's next-to-last game, then served out the victory at love.
"I think the guy's beat a brick wall once", Millman said, "because he makes you work hard for every point and it's relentless".
The Croatian has built on that success, reaching two more finals in the last two seasons, but Nishikori's only similar run came in NY two years ago when he defeated Andy Murray to reach the semi-finals before losing to Stan Wawrinka.
Only once in the professional era that began in 1968 had Japan had a men's and women's player in the quarterfinals at the same tournament.
Osaka becomes the first Japanese woman in a Grand Slam semifinal since Kimiko Date at Wimbledon in 1996.
Each man blew chances to gain control Wednesday and perhaps have a much quicker ending to a match that had wild swings of momentum.
The No. 21 ranked Nishikori defeated No. 7 ranked Cilic in five sets, 2-6, 6-4, 7-6, 4-6, 6-4.
She was quickly down two more breaks at 4-0 before squandering three break points in the sixth game as Osaka glided into a maiden semi-final at the Slams.
She's also the first Japanese woman to make the US Open semifinals in 22 years. He faces Djokovic next.