Preston upstaged higher-ranked American Alycia Parks 6-3 6-3 in just 67 minutes to record her maiden grand slam win outside of Australia.
"It's really special," the 20-year-old said after the straight-sets demolition job vindicated Tennis Australia's decision to award the tyro entry to the final major of 2026 under a reciprocal arrangement with the USTA.
"Glad that my first grand slam win was in Australia, but hopefully I can keep winning rounds at all the slams, not just at home."
Victory earned Preston a minimum $US190,000 ($A$265,000) pay day and a second-round shot at either another US hope, Taylor Townsend, or Polish 21st seed Maja Chwalinska, this year's surprise French Open runner-up.
Preston cracked the world's top 100 last week for the first time and her shooting star continues to shine under tennis's brightest lights.
She is projected to soar to another fresh career-high of No.89 in the world even with a second-round loss.
While Preston was elated, Talia Gibson was crestfallen after allowing Donna Vekic to crash her Open party amid an otherwise rough start for the Australian contingent.
Storm Hunter and Adam Walton also made early exits while Kim Birrell and Aleksandar Vukic both stood on the brink of elimination before rain suspended their matches for the night.
Gibson was unable to disguise her anguish at an opportunity lost.
The 22-year-old rallied from a set and a service break down to look poised to also claim a breakthrough maiden win at a major outside of Melbourne before Vekic steadied to post a 6-2 4-6 6-3 in two hours and 33 minutes.
The veteran Croat and 2024 Wimbledon semi-finalist seemed to be unravelling after blowing a set and 4-1 lead, cursing herself and berating her box as Gibson roared back into the contest.
But Gibson was unable to wrestle back control after failing to convert any of four break-point chances in the second game of the deciding set.
"It's a hard one to swallow," Gibson said.
"Most of the time it's who makes the most unforced errors on the court who comes away with the loss and today that was me, so it's just something to learn from."
But after starting the year well outside the top 100, Gibson has slashed her ranking in half after racking up some 30 match wins, including one over former No.1 and dual Australian and US Open champion Naomi Osaka.
"Days like these are really tough. It's hard not to be disappointed. When it comes to the slams, every player wants to do well," said the world No.63.
"But if I'm looking at my entire year so far, I've been able to achieve some things that probably at the start of the year I wouldn't have thought I'd be able to do."
Walton missed two set points in the third set of a tight 7-6 (7-2) 6-2 7-5 loss to China's Yibing Wu before Hunter - after a taxing run to become the first Australian woman to qualify for all four slams - ran out of gas in a 6-4 6-1 defeat to 11th-seeded Ukrainian Marta Kostyuk.
Japan's Rei Sakamoto was serving at 4-3, 30-0 in the fifth and deciding set against Vukic as rain washed out play for the rest of the night on the outside courts.
And Birrell was down a set and a mini break in the second-set tiebreaker against Croatian Petra Marcinko when they were forced off, needing to return on Monday (Tuesday AEST).
World No.7 and three-time US Open quarter-finalist Alex de Minaur opens his title quest on Tuesday (Wednesday AEST) against Italian qualifier Andrea Guerrieri.
This AAP article was made possible by support from Tennis Australia.
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