Early morning fog rolled in off the Pacific Ocean to delay the start of the 121st US Open by 90 minutes on Thursday before American Sahith Theegala hit the opening shot from the first tee.

A field of 156 will challenge the formidable 7,652-yard, par-71 oceanside course, with local hero Phil Mickelson trying to complete a career Grand Slam and American Bryson DeChambeau looking to defend his title.

Mickelson, ranked 30th, was scheduled to start at 9:21 a.m. (1721 GMT) off the 10th tee alongside two other Southern California players, Max Homa and sixth-ranked Xander Schauffele.

With sunset set for 7:58 p.m. and the final groups originally scheduled to finish only about 15 minutes earlier, the delay may see the day end with players still on the course.

Such stars as fifth-ranked DeChambeau, four-time major winner Rory McIlroy, world number one Dustin Johnson and third-ranked Jon Rahm could be racing the darkness to complete their rounds.

Mickelson, who turned 51 on Wednesday, became golf's oldest major winner last month by taking the PGA Championship at Kiawah Island to capture his sixth career major title.

But the US Open is the crown that has eluded him, the left-hander settling for a record six runner-up showings at the event.

With a victory, Mickelson would join a select group of career Grand Slam winners that includes Tiger Woods, Jack Nicklaus, Ben Hogan, Gary Player and Gene Sarazen.

Mickelson has won three times at Torrey Pines in US PGA Tour events but all were before a 2001 course renovation, forcing him to spend much of the past two weeks re-learning the nuances of a course he once knew well.

Mickelson remains a popular bet at 50-1 odds after his historic triumph at Kiawah Island, raising concerns from some oddsmakers about major payouts if he manages another shock major win.

World number one Dustin Johnson, last year's Masters champion, could be overtaken in the rankings if number two Justin Thomas or Spaniard Rahm win the US Open.

Johnson would need to finish worse than solo 17th if Thomas wins or worse than a two-way share of 18th if Rahm wins to be dethroned.

DeChambeau and four-time major winner Brooks Koepka, embroiled in a social media spat, were not paired together for the opening two rounds.

Koepka, Thomas and 2020 PGA Championship winner Collin Morikawa were set for an early 10th-tee start two groups ahead of Mickelson.

DeChambeau tees off at 2:44 p.m. with Masters winner Hideki Matsuyama and US Amateur winner Tyler Strafaci with McIlroy, Johnson and England's Justin Rose two groups behind them.