Phil Mickelson's bid to complete the career grand slam in his home city of San Diego got off to a poor – and somewhat unfortunate – start at Torrey Pines.

After play in the first round of the 121st US Open was delayed by 90 minutes due to fog, Mickelson covered his opening six holes in three over par.

Starting on the back nine of the South Course, 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.

Just nine days before rolling back the years to win the US PGA and become golf's oldest major champion, 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the left-hander has recorded just four top-10 finishes and in the last seven years has missed the cut three times, withdrawn once with a back injury, skipped one year to play on the European Tour and has a best finish of 14th in 2017.

Two-time winner Brooks Koepka was part of a five-way tie for the early lead on two under, with England's Tyrrell Hatton and Tommy Fleetwood a shot behind.