Phil Mickelson's "unique opportunity" to complete the career grand slam in his home city of San Diego came to a strange end before the battle for the 121st US Open intensified at Torrey Pines.

Mickelson began the week with renewed hope of winning his national open for the first time, but ended it by hitting his approach to the 18th around 70 yards beyond the hole and completing a 75 to finish 11 over par, almost two hours before the final group was due to tee off.

The 51-year-old was disappointed not to have played better, but remains confident he can replicate the form which made him the oldest major championship in last month's US PGA Championship.

"There's some opportunities coming up with the way I've been playing that I'm optimistic that I can compete and contend," the six-time major winner said.

"There's nothing more fun for me than to be in it on the weekend. I've actually been playing well enough to have chances, and we have some good tournaments coming up the next couple of months.

"Afterwards I'll look back and reminisce when the season's over and I'll still have that Wanamaker trophy I'll be looking at, and I'm still looking to add a friend to it along the line.

"That win was very meaningful to me because I've been putting in a lot of work the last couple of years and getting nothing out of it, and so to have a moment like that is something that makes it worthwhile."

While Mickelson departed the scene, Rory McIlroy was firmly focused on winning the title, a decade since his breakthrough victory at Congressional.

The 32-year-old cruised to an eight-shot victory in 2011, just two months after blowing a four-stroke lead in the Masters following a final round of 80 at Augusta National.

The Northern Irishman went on to win the US PGA Championship in 2012 and the Open and US PGA in 2014, but has not won a major since and had been a combined 35 over par in the first round of golf's biggest events before an opening 70 in San Diego.

A second round of 73 left McIlroy six shots off the pace at the halfway stage, but a 67 on Saturday lifted him to three under par, just two behind leaders Louis Oosthuizen, Mackenzie Hughes and Russell Henley.

McIlroy did well to make par on the first after finding sand off the tee and hitting a conservative recovery to the front of the green, but then holed a curling birdie putt from 35 feet on the fourth.

At four under he was just one off the lead held by American Henley, with defending champion Bryson DeChambeau, Oosthuizen and Hughes alongside McIlroy and Jon Rahm and Brooks Koepka another stroke back.