Sweeny complained of a hip injury during a 6-4 6-3 6-3 defeat to the world No.22 that left Australia without a second-week singles hope in New York for the first time in five years.
Alex de Minaur's shock second-round loss to Botic van de Zandschulp had left Sweeny as Australia's last man standing at Flushing Meadows.
"It's been an amazing week, a career-changing week, but I can't help but feel a bit disappointed just with today's performance," Sweeny said.
"I just wasn't really feeling it, just not really feeling the timing and the rhythm as I have been. Just made a lot of cheap errors and that was quite frustrating.
"I was OK to lose a match, but I really wanted to test him more and bring a good level so I could make an absolute fight out of it. Win or lose, I wanted to really make him work for the victory at least and I wasn't able to do that."
The world No.123 only gained a last-minute entry to the season's last slam after two late withdrawals that allowed Tennis Australia to grant Sweeny the reciprocal wildcard they initially handed to Alexei Popyrin.
And despite his disappointment, Sweeny cashed in on his good fortune, the 25-year-old parlaying his maiden US Open main-draw appearance into a watershed run to the last 32 of a grand slam for the first time.
The Queenslander will walk away with a life-changing cheque for $US290,000 ($A402,500) and a fresh career-high ranking.
"The financial side of things, that will help with coaching and people you travel with," Sweeny said.
"I'm always investing back in my tennis. That's the No.1 goal, to do all I can to be the best player I can be, so money I don't really worry about spending on my tennis."
Sweeny is provisionally slated to climb to No.109 in the world.
If he can crack the top 100 by season's end, Sweeny will earn direct entry to the Australian Open for the first time.
"That's exciting too. But that will take care of itself," Sweeny said.
"It's hard not to to worry about that because we're indoctrinated to want that low number next to our name, but I can just focus on working hard, focus on improving.
"Particularly just staying really kind to myself and trying to enjoy the journey."
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