Masters champion Jon Rahm's bid for the third leg of the career Grand Slam of golf's four majors got underway on Thursday at the PGA Championship where frost delayed the start of the first round by nearly two hours.

Spanish world number one Rahm set off in the ninth group off the 10th tee alongside U.S. Open champion Matt Fitzpatrick and Australian Cameron Smith at sunny Oak Hill with temperatures around 45 Fahrenheit (7.2 Celsius) at the year's second major.

Brooks Koepka, who along with fellow LIV Golf member Phil Mickelson finished joint runner-up at the Masters, was playing three groups ahead in the company of world number two Scottie Scheffler and Gary Woodland.

That trio was being followed around the course by another high-profile group made up of defending champion Justin Thomas, world number three Rory McIlroy and twice major champion Collin Morikawa.

Thomas matched the largest fourth-round comeback in PGA Championship history last year when he overcome a seven-shot deficit at Southern Hills before emerging as the winner in a three-hole aggregate playoff.

Japan's Kazuki Higa, making his PGA Championship debut, grabbed the early on-course lead with a scorching start that saw him cover his first five holes in four under par.

With early-morning temperatures at 28 Fahrenheit, Oak Hill was left coated in a thin layer of white until mainly sunny skies brought a more familiar look to the course.

Oak Hill has undergone a restoration since it last hosted a PGA Championship in 2013 and this week the par-70 layout will present a challenging tee-to-green test featuring narrow fairways, deep rough and steep-faced bunkers.

Tournament favourite Rahm, who won the 2021 U.S. Open and added a second major title last month at Augusta National where he triumphed by four shots, is looking to become only the sixth golfer in the last 70 years to win the year's first two majors.

Jordan Spieth, making his seventh attempt at completing the coveted career Grand Slam of golf's four majors, also went out early from the 10th tee alongside Ireland's Shane Lowry and Norway's Viktor Hovland.

Spieth arrived at Oak Hill facing questions about his fitness after a left wrist injury forced him to withdraw ahead of last week's PGA Tour stop in Texas.

Mickelson returns to the PGA Championship this year for the first time since 2021 when, aged 50, he defied the odds to become the oldest major winner in history.

The six-times major winner skipped his title defence last year at Southern Hills as he extended his self-imposed hiatus from the sport after incendiary comments he made about the Saudi-funded rival league which became LIV Golf.

Mickelson is scheduled to tee off in the 3:48 p.m. (1948 GMT) group from the 10th tee with fellow Americans Patrick Cantlay and Rickie Fowler. (Reporting by Steve Keating in Rochester and Frank Pingue in Toronto; Editing by Ken Ferris and Toby Davis)