"Felt like a deer in the headlights to begin with, to be honest", he said courtside.
Before the match, Federer said he was "a big fan of John", having trained with him in the past.
"I managed to play on some of the biggest courts now: Wimbledon Centre Court and Australian Open, Rod Laver Arena".
"As always. (in a) match against Keys, (it's) hard for me to play my game and to get into the rhythm and just do what I do best on court".
The 14th-seeded Keys will play Carla Suarez Navarro who ended Maria Sharapova's flawless record in night matches at the U.S. Open, reaching the quarterfinals with a 6-4, 6-3 victory.
"I'm not 21 anymore".
"You don't get an opportunity to hit around the net post very often".
For his troubles, John Millman - whose rabid fanbase are called Millminions for what it's worth - now faces another huge task in sixth-seeded Novak Djokovic in his quarter final matchup.
Since then, nine Australians - including Nick Kyrgios only two days ago, Mark Philippoussis in the 2003 Wimbledon final and Lleyton Hewitt seven times - had fallen prey to Federer in 20 straight grand slam matches over nearly 20 years.
The first Portuguese player to reach the fourth round of any Grand Slam, the 68th ranked Sousa never looked a threat to progress any further as his magical U.S. Open run came to an end.
Djokovic beat Joao Sousa 6-3, 6-4, 6-3 to move into the final eight at Flushing Meadows for the 11th time. After reaching a career-high fourth in the world in 2015 he slid down the rankings as a wrist injury sidelined him for six months, keeping him out of last year's US Open and eventually requiring surgery.