THE only time John Millman played Roger Federer, he left the experience feeling disappointed, even if he was just about the only one who felt that way.

Millman will have his greatest tennis payday, a guaranteed $A370,000, even if he loses his US Open fourth-round match against Federer on Tuesday morning (AEST).

The Brisbane right-hander was ranked No.156 at the 2015 Brisbane International when he led Federer by a set and 3-1, with a point on his serve to move to 4-1, before losing 4-6, 6-4, 6-3.

“I was very disappointed and upset with that result, but not discouraged,’’ Millman told The Courier-Mail in January.

“If I can stay injury-free, I know that I can compete at a very high level.’’

Millman vowed, after his tenacious 6-4, 4-6, 6-1, 6-3 third-round triumph over Mikhail Kukushkin today, that he would not be daunted by the reputation of the 20-time Grand Slam title.

It will likely be his first match on Arthur Ashe Stadium, the biggest in world tennis.

Millman, 29, will have a career-best ranking after the US Open, moving to at least No.42 on the basis of his longest run at a Grand Slam event, seven spots better than his best, achieved last month.

The Queenslander spent several days training with Federer at the Swiss star’s grass court base before Wimbledon.

Millman has overcome several major injury challenges, having surgery twice on his shoulder and a groin operation last year which, just over a year ago, caused his ranking to fall to 235.

“This is the biggest moment of my career,” he said in New York.

“It’s been a bumpy road. Each time I’ve come back from (the shoulder) surgery I had extreme doubts over whether I would get back.

“I’ve got a lot of respect for Roger. He’s got an aura about him. Definitely a player I’ve looked up to throughout my career.

“But I have never been a fan of playing anyone’s reputation. I want to start off at love-all and not already being behind the eight-ball and playing someone’s reputation.

“I will relish the opportunity.”

Millman should have cast off his “tennis battler’’ tag with a 2018 season that featured his first win over a top-15 player (Lucas Pouille) and a maiden ATP final (which he lost) that swelled his purse to $US443,743 before the US Open.

“If I had played tennis for the money, I would have quit a long, long time ago because I have come from the scraps just about on the tour,” he said.

“For me, it was more about being able to create that opportunity to put myself in a position where I am playing second week of a Grand Slam. That is very special.

“It has to do with just having an extended period of being healthy. It doesn’t come as a surprise to me that, you know, I get 12-14 months on the trot that my game and my ranking has gotten better.

“It’s about trying to stay fit for me. It hasn’t come that easy. I feel like I do all the right things, but sometimes it’s not meant to be with my body.”