Mickelson won his third Masters at Augusta in April
Phil Mickelson got off to a poor start in his quest to win a first US Open title with a four-over 75 in his first round at Pebble Beach on Thursday.
The Masters champion leaked three straight shots from the 16th after taking drops on the 17th and 18th.
The 40-year-old, who started at the 10th, made another bogey on the fourth and ended without a single birdie.
Mickelson, runner-up last year for the fifth time, could overtake Tiger Woods as world number one if he wins.
The three-time Masters champion failed to take advantage of ideal scoring conditions in California as he notched up his highest US Open first round since taking 75 at Congressional in the 1997.
Mickelson missed back-to-back birdie putts of five feet at the 14th and 15th before a run of bogeys.
At 17 Mickelson found the beach and took a drop on the way to a bogey, while at 18 the left-hander's tee shot left him blocked by two trees. He tried to play a three-wood draw around them but the shot drifted out, bounced on the rocks and raced into the Pacific Ocean.
He missed 10-foot birdie chances at the second and third holes and also slid a three-foot birdie putt past the outside edge at the par-five sixth.
"It was terrible on the greens, one of my worst putting days," said Mickelson. "I misread a lot and hit a bunch of poor putts and didn't make birdies because of it.
"It's disappointing. It was a very fair set-up, conditions were good and it was under-par scoring today."
Mickelson, who went straight to the practice green after his round to work on his putting, said the dry weather forecast would make the course play even faster and harder.
"We have three rounds left. I know if I shoot under par tomorrow I'll be right there," he said. "There's no way under par is going to win here, I don't believe. I think over par will win. Because of that, I'm right there."
Mickelson would also move to the top of the rankings if he finishes second on his own and Woods finishes outside the top four.
Should Woods make the cut but finish outside the top 18, Mickelson would need to finish no worse than third on his own, and if Woods misses the cut, his fellow American would need to finish no worse than a two-way tie for third to lead the rankings.