In addition to this, the 20-time Grand Slam champion also revealed that he struggled to breathe and couldn't get any air.
Then Millman won the third-set tiebreaker and the crowd remembered why they were there.
Roger Federer surveyed the wreckage of his US Open exit at the hands of John Millman, insisting there was "no shame" in defeat and challenging the wisdom of anybody fearless enough to start penning his career obituary.
Though he wouldn't say it afterward, Federer clearly wasn't right during the Millman match.
Sharapova, seeded 22nd, had been 22-0 under the lights in Arthur Ashe Stadium.
Venue: Flushing Meadows, New York Dates: 27 August-9 September Coverage: Live radio coverage on BBC Radio 5 live sports extra; live text commentaries on the BBC Sport website. Cilic beat Nishikori in the 2014 final.
Tsurenko struggled in the heat.
The first obvious signs of trouble for Federer came far earlier, in the second game of the second set. She still and looked tired to start the second, leaning on her racket between points while falling behind 2-0. "I felt I couldn't get any air and I was sweating more and more and became more and more uncomfortable", explained Federer. "It's a late finish, but I'll try to get plenty of sleep, have a bit of a light hit tomorrow afternoon, get the body in good order". In the following tiebreaker, he double faulted twice in a row to hand his opponent a 4-1 lead.
Seeing him lose a second consecutive final to Novak Djokovic at Wimbledon - the Grand Slam that has brought him the most success - I puffed out my chest and declared Federer as finished. Big men John Isner and Juan Martin del Potro duel in a rematch of their Miami semifinal won by the American and French quarter taken by Delpo.
Osaka will next face Lesia Tsurenko, the 36th-ranked Ukrainian who beat Czech teenager Marketa Vondrousova 6-7 (3/7), 7-5, 6-2.
At last year's US Open, Millman bowed out in the third round.
Stream every match at the US Open live on the FOX+ app and catch up on highlights of any action you've missed. The German reached his lone Grand Slam quarterfinal at Wimbledon in 2012.
Serena Williams powers into the US Open semi-finals
After just 20 minutes Tuesday night, Williams was in danger of trailing by two service breaks. Stephens yelled in one moment of frustration, just minutes after the U.S.
In a match played late into the night in NY, world No.55 Millman progressed to the last eight with a 3-6 7-5 7-6 (9-7) 7-6 (7-3) victory.
Keys was an Open finalist a year ago and lost to Sloane Stephens.
The match went late into the night in NY.
In all he managed to score 47 winners compared to Federer's 65, but he conceded only 28 unforced errors.
The extreme heat policy is back in effect at the U.S. Open.
Federer was 4-2 up in the fourth set but was reeled in and two back-to-back double faults sent him 1/4 down in the tiebreak. U.S. Open officials began putting it in play last week, when temperatures first soared into the mid-90s.
It was 87 degrees just before the start of Monday's day session on Arthur Ashe Stadium.
In this phenomenal era, and over five sets in the sapping humidity of NY, we should not be at all surprised Federer has fallen short but we are allowed to feel a little glum.
Up to that point Federer hadn't lost a set all tournament as he had his eye on a sixth US Open crown. "It's 1 o'clock now", John said during his on-court interview after defeating Roger Federer.
"He pushed me around, " Kohlschreiber said after Nishikori reached the final eight for a second straight Grand Slam tournament.
Madison Keys was in dominant form in her women's singles fourth round win over Dominika Cibulkova.