To complete the patriotic all-American look, the flamboyant Queenslander sported a quarter-zip jacket that featured a blue and white star-lined collar.
Day is one of six Australians in the event, with their challenge to be led off by Adam Scott when the 125th US Open starts just outside Pittsburgh on Thursday.
Scott will tee off at 7.18 am local time (2118 AEST) alongside Sweden's Ludvig Aberg and Hideki Matsuyama, of Japan.
The veteran's best finish was a tie for fourth in 2015, two shots adrift of winner Jordan Speith.
Cam Smith, who also tied fourth in 2015, a finish he repeated in 2023, is another early starter paired with fellow former British Open winner Brian Harman and LIV Golf colleague Phil Mickelson, who still needs this major to complete a grand slam.
Smith goes off at 8.02 (2202 AEST) from the 10th, just as Cam Davis is teeing off at the first. He is with American Davis Thompson and Thomas Detry of Belgium.
Day, Min Woo Lee and Marc Leishman start in the afternoon.
Lee is in one of the higher-profile groups with major winners Justin Thomas and Brooks Koepka, who won the US Open in 2017 and 2018.
Matt Vogt, a former Oakmont caddie who now works as a dentist, but qualified as an amateur last week, will hit the opening tee shot at at 6.45 am local time.
Defending champion Bryson DeChambeau is grouped with two-time major winner Xander Schauffele and reigning US Amateur champion Jose Luis Ballester, who recently joined DeChambeau on the LIV Golf circuit.
World No.1 Scottie Scheffler, winner of last month's PGA Championship at Quail Hollow, shares the tee box with two-time major winner and World No.4 Collin Morikawa and Norway's Viktor Hovland.
World No.2 and 2025 Masters champ Rory McIlroy of Northern Ireland will play with Ireland's Shane Lowry and England's Justin Rose. Rose (2013) and McIlroy (2011) are former US Open champions and Lowry tied for second in 2016 - the last time the event was held at Oakmont, when Dustin Johnson won.
He is grouped with Spieth and 2021 champion Jon Rahm of Spain.
US Open: selected tee times
First hole - 10th hole (Thursday/Friday unless stated). All Times AEST
2129 & 0314 (Sat): Xander Schauffele (US), Jose Luis Ballester (Spain), Bryson DeChambeau (US).
2202 & 0347 (Sat): Cam Davis (Australia), Davis Thompson (US), Thomas Detry (Belgium).
0303 (Fri) & 2118: Min Woo Lee (Australia), Justin Thomas (US), Brooks Koepka (US).
0325 (Fri) & 2140: Viktor Hovland (Norway), Collin Morikawa (US), Scottie Scheffler (US).
0336 (Fri) & 2151: Corey Conners (Canada), Jason Day (Australia), Patrick Reed (US).
10th hole & first hole (Thursday/Friday unless stated)
2118 & 0303 (Sat): Ludvig Aberg (Sweden), Adam Scott (Australia), Hideki Matsuyama, (Japan).
2140 & 0325 (Sat): Shane Lowry (Ireland), Justin Rose (England), Rory McIlroy (Northern Ireland).
2202 & 0347(Sat): Cameron Smith (Australia), Brian Harman (US), Phil Mickelson (US).
0314 (Fri) & 2129: Jordan Spieth (US), Jon Rahm (Spain), Dustin Johnson (US).
0358 (Fri) & 2213: Marc Leishman (Australia), Aaron Rai (England), Nick Dunlap (US).
with agencies
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