U.S. Open tournament director David Brewer says the tournament's hard courts were purposely slowed down "a touch" in response to players noting in recent years that the surface seemed to be speeding up.
The Wimbledon champion has appeared more impressive with each passing round and will be up against a confident Millman who is unseeded but flying high after stunning Swiss second seed Federer in the previous round.
Djokovic said it was fine, because he needed a bit of a rest. "It's been pretty hot here, and it can be - I don't think, whatever the humidity is on the outer courts or in the city, I think it's amplified on centre court".
Keys won all 10 of her service games, saving the only two break points she faced.
"We were both struggling and trying to hang in there".
"I was struggling, he was struggling, we were all sweating, changing clothes".
Djokovic won the first set of the match 6-3 but started to have an issue early in the second. Federer had the advantage in the shortest and mid-range points while Djokovic erased that deficit with the better performance in the longer rallies, saving many of those break points after drawing Roger into more extended exchanges where he had the upper hand.
Osaka wasn't born when compatriot Kimiko Date made the Wimbledon semi-finals in 1996 but now the 20-year-old has the opportunity to go one better by reaching a maiden Grand Slam final.
But Djokovic broke back in the match's next-to-last game, then served out the victory at love.
She raced to a 3-0 lead in the first set and then 4-0 in the second against the shaky Tsurenko, who finished with more unforced errors than points in her first major quarterfinal.
But the 13-time major winner's class ultimately told, gaining a decisive break in each set to advance to a last-four meeting on Friday with rejuvenated 2014 runner-up Kei Nishikori. He broke Djokovic's serve only once and posted just two break points.
Meanwhile, Australian Open champion Caroline Wozniacki called out supporters in the stands at the Miami Open earlier this year for abuse suffered by both her and her supporters in her player box.
He showed a resemblance of a fight in the third set, breaking Djokovic who double-faulted on the back of two serve-time violations.
The popular Queenslander said he copped online abuse after his first-round loss to Taro Daniel at Winstom Salem the week before the US Open.
It was already 85 degrees (29.4 Celsius) outside of Arthur Ashe Stadium about a half hour before the opening quarterfinal match between No. 20 seed Naomi Osaka of Japan and Lesia Tsurenko of Ukraine. Yet the Wednesday-night encounter was anything but straightforward.
The No. 20 seed moved from Japan to NY at age 3, and her deepest major run is coming at the same tournament she first visited as a child.