Keys, who at 14 is the highest seed left in the women's draw, has shown a determination to go one step further than previous year, having only dropped one set through her four matches. "Credit to John, he put up a great battle". He's a great fighter.
"He's had an fantastic tournament, first quarterfinals. I've had three very significant surgeries, two on the shoulder and one on the groin", Millman told the Today show.
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 unsafe. "It's easier said than done, especially in these kind of conditions".
For the men, the humidity means packing 10 or 11 shirts to keep up with the sweat and changing shoes on court, as Dominic Thiem did in his epic four-hour, 49-minute quarterfinal loss to Rafael Nadal.
"I was just trying to hang in there and try to find a way to win the match". There was a lot of sweating but you just need to survive.
After Suarez Navarro pulled off a stunning upset win over Maria Sharapova earlier in the week, she was eventually overpowered and outmatched by Keys in Wednesday's quarterfinal.
And he gave them enough reasons to cheer.
"There are so many people that stayed at the stadium and its past midnight".
"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".
Wheelchair tennis has been around for more than four decades with 120 professional events in 80 countries, but only recently become a mainstay all four Grand Slams.
The crisis soon deepened for Tsurenko, who had needed the best part of three hours on Monday to beat Czech teenager Marketa Vondrousova in a match where she nearly collapsed due to heat exhaustion.
Kei Nishikori advanced to the U.S. Open semi-finals on Wednesday with a grinding 2-6 6-4 7-6 (5) 4-6-6-4 win over Marin Cilic, gaining a small measure of revenge on the man who crushed his grand slam dream in the 2014 final.
Djokovic, now ranked at number 6, defeated our last remaining hope for an Australian victor 6-3, 6-4, 6-4 in 2 hours and 49 minutes.
"I felt I couldn't get air", Federer said.
Francesca Schiavone has retired from tennis after a career in which she won the French Open title and three Fed Cup championships with Italy. "I think it would be really cool".
"I find it a bit unusual, but obviously they lack a little bit of character".
But she will face a stern test from Suarez Navarro, who is brimming with confidence after arriving in NY on the heels of a finals appearance at New Haven.
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.