The unheralded, 170cm-high wildcard is Australia's last man standing in New York after winning a real-life David-versus-Goliath battle to rock the Open with a stunning second-round upset over dual grand slam semi-finalist Lorenzo Musetti.
As world No.7 Alex de Minaur spectacularly crashed out on Thursday (Friday AEST), Sweeny took down the 14th-seeded Musetti 3-6 6-1 6-2 6-2 for the biggest win of his career.
'Teeny' Sweeny rose to the occasion to reach the third round of a grand slam for the first time and earn a minimum pay day of $US290,000 ($A402,750).
"It feels great. It's exciting. I'm stoked," Sweeny told AAP after backing up his maiden grand slam victory over retiring French great Gael Monfils at this year's Australian Open with another big scalp.
"To be in the third round, I'm really happy with obviously winning, but more so how I stayed really composed and just really trusted myself in the big moments."
Fast becoming a fan favourite at Flushing Meadows for his laid-back demeanour and the way he throws himself about the court with little preservation for his body, Sweeny says he was more in control trying to close out his second-round match than playing at home with friends.
"Well, me and my mates actually have a yearly wooden racquet tournament and I'm probably twice as nervous for that as I am to play a grand slam," said the 25-year-old battler.
"So that's put me in good stead to be able to stay composed on the grand slam level.
"Honestly, I'm in the third round of the US Open, but I lost to a guy (in the wooden racquet tournament), Vincent Giuffre, who I don't know if he ever got an ATP point, but he beat me 4-0 last year.
"So I'm going to come into next year's invitational unseeded, which is dangerous for the other boys."
Ranked more than 100 places below his world No.14 opponent, Sweeny bossed Musetti into submission.
He ran the world No.14 and former Wimbledon and French Open semi-finalist ragged.
Despite sporting a massive ice pack on his right elbow after his first-round win, Sweeny continued diving like Boris Becker for acrobatic winning volleys at the net.
The 25-year-old's reward is a last-32 shot at another Italian, world No.22 Luciano Daredri on Saturday (AEST) - and a rankings rise to a career-best No.115 in the world, or better.
"I'm just grateful to be even able to compete in grand slams. The tennis journey is such a rollercoaster," Sweeny said.
For a couple of heady hours it looked like his underdog countrymen Tristan Schoolkate and Alexei Popyrin would join Sweeny in round three.
But Schoolkate's inspired run from qualifying finally came to a halt at the hands of fifth-seeded French Open runner-up Flavio Cobilli.
Schoolkate snatched the first set before the Italian showed his class to prevail 3-6 6-3 6-4 7-5 after two hours and 41 minutes in New York's scorching summer heat.
"I gave it my best shot, threw the kitchen sink at him," Schoolkate said.
"I pulled up second best today, but pretty proud of my efforts over the past couple of weeks."
And two years after upstaging Novak Djokovic in the US Open's third round, Popyrin fell short of a place in the last 32 once again, losing a tense four-set battle with Chilean Alejandro Tabilo.
Like Schoolkate, Popyrin took the opening set before the 25th-seeded South American claimed two pivotal tiebreakers, then the match 2-6 7-6 (7-4) 7-6 (7-3) 6-2 to seal a likely third-round date with German top seed Alexander Zverev.
With de Minaur slumping to a shock 6-4 7-5 6-2 loss to big-hitting Dutch world No.70 Botic van de Zandschulp, the 123rd-ranked Sweeny is Australia's last singles hope at the Open.
He's placing no limit on how far he can go.
"That's in God's plan. I have no idea," he said.
"So I'll just keep doing my best, just keep trying to play good tennis and enjoy the moment, enjoy the whole experience and I'm not sure, we'll see."
