The Australian said he froze against Federer in the first set -"felt like a deer in the headlights to begin with,to be honest"- but was able to take control by relying on his fighting qualities against the 20-time grand slam singles champion.
"I'm probably in a little bit of disbelief. I have so much respect for Roger and everything he's done for the game,"Millman told ESPN after the match.
Loading
"He's a hero of mine. Today he was not at his best but I'll take it."
Millman may not be a huge name in men's tennis currently,but Federer said he was an admirer before the pair's clash.
"I'm a big fan of John,"said Federer,who was chasing a sixth US Open title."Maybe not that many people know him,but he's a real hard-working guy out there and those are the guys who have all the respect from us top players in the locker room.
"He's just come to Switzerland to train with me before the grasscourt season so we know each other quite well,a super friendly guy,and I'm very happy for him that he's gotten so far."
Before his match against Federer,Millman had amassed more than $US443,000 ($614,000) in prizemoney this year,for a career total of $US1.85 million. In late July,he entered the world's top 50,climbing to a career-high ranking of No.49.
Not since Pat Rafter handed a tantrum-throwing Federer a tennis lesson on the then-18-year-old's grand slam debut at Roland Garros in 1999 had an Australian conquered the great Swiss at a major.
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 almost 20 years.
But on the very same court on which Federer destroyed Hewitt in straight sets in the 2004 US Open final,Millman - who resides in his birthplace of Brisbane - stared down the greatest player of the professional era to send Federer crashing to his earliest grand slam defeat in almost four years.
"I got out of a tough second set and really found my feet and started to be a bit more aggressive,"Millman said."I started to serve really well and probably capitalised a little bit on Roger having an off service day."
with AAP