Brooks Koepka made an impressive start to his bid for a third US Open victory in five years as Phil Mickelson's hopes of completing the career grand slam suffered a significant blow.
Koepka, who won the title in 2017 and 2018 and was second in 2019, carded an opening 69 at Torrey Pines to lie two shots behind early clubhouse leader Russell Henley.
Former Open champion Francesco Molinari was a shot off the lead following a 68, but Mickelson was four over par approaching the end of his round, a month after becoming the oldest champion in major history with his shock victory in the US PGA Championship.
Starting on the back nine, Koepka birdied the 12th, 17th and 18th to reach the turn in 33 before another birdie on the second took him into the outright lead on four under.
A wild tee shot to the third cost the 31-year-old his first bogey of the day and another shot went on the seventh after a pulled drive which finished at the base of a picnic bench where spectators were eating lunch.
Also starting on the back nine after a 90-minute delay due to fog, Mickelson dropped a shot on the 10th when he failed to get up and down from a greenside bunker, his par attempt catching the edge of the hole and spinning out.
The six-time major winner was even more unfortunate on the 13th when, after taking a penalty drop from a bush, his fourth shot to the 613-yard par five bounced once before hitting the pin and rebounding off the green.
Mickelson did well to limit the damage to a single shot by chipping to six feet and holing the putt, but another shot went on the 15th after an errant drive.
A birdie on the 17th stopped the rot and Mickelson also found the green on the par-five 18th in two, but could only three-putt from 60 feet for par.
Just nine days before rolling back the years to win the US PGA, Mickelson had reluctantly accepted a special exemption for the US Open, an event in which he has finished runner-up a record six times.
The exemption was required because Mickelson had slipped out of the world's top 100 and the prospect of him joining Gene Sarazen, Ben Hogan, Gary Player, Jack Nicklaus and Tiger Woods in having won all four majors looked increasingly far-fetched.
However, Mickelson's stunning victory at Kiawah Island came with a five-year US Open exemption and the 51-year-old – who celebrated his birthday on Wednesday – spent last week practising hard at a course he used to love.
Mickelson has won three times at Torrey Pines, including back-to-back victories in the Buick Invitational – now the Farmers Insurance Open – in 2000 and 2001, but three months after the last of those successes the South Course underwent a major redesign which was not to Mickelson's liking.
In his 18 starts since, the left-hander has recorded just four top-10 finishes and in the last seven years has missed the cut three times, withdrawn once with a back injury, skipped one year to play on the European Tour and has a best finish of 14th in 2017.
England's Marcus Armitage made an impressive start on his US Open debut with a level-par 71.
