San Francisco - Andy Zhang of China, aged 14, was set to become the youngest competitor at a US Open since World War II after American Brandt Snedeker and Britain's Paul Casey withdrew from the event on Monday due to injuries.
Zhang, who attends school in the United States, gained his spot in the 156-strong field at the Olympic Club when Englishman Casey pulled out because of a shoulder injury.
American Tadd Fujikawa was previously the youngest player since World War II at a US Open - aged 15 years, five months and seven days during the 2006 edition at Winged Foot.
US Open records are not fully comprehensive before 1945, but United States Golf Association officials said Zhang was believed to be the youngest player ever at the championship.
Casey, who dislocated his shoulder while snowboarding in January, has competed in only seven events worldwide this year, missing the cut in five of them.
"You're always disappointed to miss time due to injury but it's especially true when you have to withdraw from the US Open," Casey said in a statement.
"I've been trying to play my way back from my shoulder injury, and I feel like I probably came back too soon originally and it's affecting my ability right now to be ready for the test of a major.
"I do feel like I'm very nearly ready but not quite ready for this week," added Casey, whose world ranking has dropped from 20th to 58th since he suffered the injury.
Earlier on Monday, three-time PGA Tour champion Snedeker withdrew from the year's second major due to a rib injury.
The 31-year-old Snedeker, who clinched his most recent title on the US circuit at the Farmers Insurance Open in January, will be replaced by American amateur Jordan Spieth.
Two-time former US junior amateur champion, Spieth will tee off with compatriots Bill Haas and Nick Watney in Thursday's opening round.
Pairings for the first round of the US Open golf championship, which
starts Thursday at The Olympic Club's 7 170-yard, par-70 Lake Course (all times SA):First Tee
16:15 - Scott Langley (USA), Steve Lebrun (USA), Beau Hossler (USA)
16:26 - Jason Bohn (USA), Raphael Jacquelin (FRA), JB Park (KOR)
16:37 - Michael Thompson (USA), Colt Knost (USA), Steve Marino (USA)
16:48 - Brendan Jones (AUS), George Coetzee (RSA), Gregory Bourdy (FRA)
16:59 - Patrick Cantlay (USA), Jonathan Byrd (USA), Kyle Stanley (USA)
17:10 - Retief Goosen (RSA), Vijay Singh (FIJ), Zach Johnson (USA)
17:21 - Andy Zhang (CHN), Hiroyuki Fujita (JPN), Mark Wilson (USA)
17:32 - Adam Scott (AUS), Keegan Bradley (USA), Webb Simpson (USA)
17:43 - Tim Clark (RSA), Toru Taniguchi (JPN), Rod Pampling (AUS)
17:54 - Francesco Molinari (ITA), Bo Van Pelt (USA), Peter Hanson (SWE)
18:05 - DA Points (USA), Lee Dong-Hwan (KOR), Kevin Streelman (USA)
18:16 - Edward Loar (USA), Paul Claxton (USA), Alistair Presnell (AUS)
18:27 - Mark McCormick (USA), Nick Sherwood (USA), Cole Howard (USA)
12:45 - Casey Martin (USA), Cameron Wilson (USA), Dennis Miller (USA)
12:56 - Jim Herman (USA), William Lunde (USA), David Mathis (USA)
21:07 - Nicolas Colsaerts (BEL), Charlie Wi (KOR), Simon Dyson (ENG)
21:18 - Alvaro Quiros (ESP), Gary Woodland (USA), John Senden (AUS)
21:29 - Rory McIlroy (NIR), Luke Donald (ENG), Lee Westwood (ENG)
21:40 - Jim Furyk (USA), Sergio Garcia (ESP), Graeme McDowell (NIR)
21:51 - Stewart Cink (USA), Trevor Immelman (RSA), Lucas Glover (USA)
22:02 - Ernie Els (RSA), Geoff Ogilvy (AUS), Angel Cabrera (ARG)
22:13 - Martin Laird (SCO), Ben Crane (USA), Anders Hansen (DEN)
22:24 - Matteo Manassero (ITA), Aaron Baddeley (AUS), Miguel Angel Jimenez (ESP)
22:35 - Brian Harman (USA), Spencer Levin (USA) Mikko Ilonen (FIN)
22:46 - Brice Garnett (USA), Justin Hicks (USA) Jesse Mueller (USA)
22:57 - Brian Rowell (USA), Alberto Sanchez (USA), Brian Gaffney (USA)
16:00 - Shane Bertsch (USA), Martin Flores (USA), Tommy Biershenk (USA)
16:11 - Scott Piercy (USA), Matthew Baldwin (ENG), Matt Bettencourt (USA)
16:22 - Thomas Bjorn (DEN), Kevin Na (USA), Branden Grace (RSA)
16:33 - Phil Mickelson (USA), Tiger Woods (USA), Bubba Watson (USA)
16:44 - Joe Ogilvie (USA), Stephen Ames (CAN), Tim Herron (USA)
16:55 - Davis Love (USA), Padraig Harrington (IRL), David Toms (USA)
17:06 - Carl Pettersson (SWE), Charl Schwartzel (RSA), Charles Howell (USA)
17:17 - Robert Karlsson (SWE), Bob Estes (USA), Robert Rock (ENG)
17:28 - KJ Choi (KOR), YE Yang (KOR), KT Kim (KOR)
17:39 - Fredrik Jacobson (SWE), Robert Garrigus (USA), Alexander Noren (SWE)
17:50 - Gonzalo Fernandez-Castano (ESP), Bae Sang Moon (KOR), Rafael Cabrera-Bello (ESP)
18:01 - Marc Warren (SCO), Anthony Summers (AUS), Michael Allen (USA)
18:12 - Hunter Hamrick (USA), Tim Weinhart (USA), Scott Smith (USA)
21:30 - John Peterson (USA), Morgan Hoffmann (USA), Aaron Watkins (USA)
21:41 - Jeff Curl (USA), Nicholas Thompson (USA), Casey Wittenberg (USA)
21:52 - Soren Kjeldsen (DEN), Chez Reavie (USA), Peter Lawrie (IRL)
22:03 - Michael Campbell (NZL), Olin Browne (USA), Joe Durant (USA)
22:14 - Bill Haas (USA), Nick Watney (USA), Jordan Spieth (USA)
22:25 - Martin Kaymer (GER), Hunter Mahan (USA), Justin Rose (ENG)
22:36 - Steve Stricker (USA), Ian Poulter (ENG), Matt Kuchar (USA)
22:47 - Jason Day (AUS), Louis Oosthuizen (RSA), Jason Dufner (USA)
22:58 - Rickie Fowler (USA), Ryo Ishikawa (JPN), Dustin Johnson (USA)
23:09 - Hunter Haas (USA), Tadahiro Takayama (JPN), Lee Slattery (ENG)
23:20 - Alex Cejka (GER), Kevin Chappell (USA), Blake Adams (USA)
23:31 - James Hahn (USA), Darron Stiles (USA), Roberto Castro (USA)
23:42 - Brooks Koepka (USA), Kyle Thompson (USA), Samuel Osborne (ENG)