US Open organisers are preparing for the likelihood of further rain and thunderstorms as the 116th edition of the year's second golf major gets under way at the treacherous Oakmont Country Club.
Under clear skies, sectional qualifier Danny McCarthy struck the first shot on Thursday on the par-4 opening hole and fellow American Andres Gonzales began proceedings at the 10th to launch the ninth US Open to be played on the par-70 layout.
More than an inch of rain had drenched the course on Wednesday night and there was an 80 per cent chance of thunderstorms later on Thursday before conditions were expected to improve over the next three days.
Northern Ireland's world No.3 Rory McIlroy was among the early starters, having been grouped with Masters champion Danny Willett of England and fifth-ranked American Rickie Fowler in one of the more eye-catching trios.
Other exciting groupings early on include defending champion Jordan Spieth and fellow Americans Zach Johnson and Bryson DeChambeau.
Spieth, seeking his third major title after clinching his first at the 2015 Masters, will aim to rebound from his stunning back-nine collapse at Augusta this year when he threw away the title after leading by five strokes with nine holes to play.
Australian world No.1 Jason Day, winner of last year's PGA Championship, has a later scheduled start time with compatriot Adam Scott and South African Louis Oosthuizen.
In other notable groupings, former No.1s Lee Westwood and Luke Donald of England and Germany's Martin Kaymer have been put together, while Japan's Hideki Matsuyama will play with Spaniard Sergio Garcia and long-hitting American Dustin Johnson.
Fan favourite Phil Mickelson, a runner-up in his national championship a record six times without winning the title, will start out with 2013 US Open champion Justin Rose of England and Sweden's Henrik Stenson.
