Pebble Beach - Jason Day, a former world number one, major winner and 12-time winner on the PGA Tour, is hoping Tiger Woods' former caddie Steve Williams can help him reignite a career in which he believes he has "severely underachieved."

"I think I've underachieved up until now," the 31-year-old Aussie said Tuesday as he prepared for the US Open at Pebble Beach. "I feel I've got a game that when it's on, I can win most tournaments.

"And the big thing for me is to go ahead and believe that, and have trust in my abilities that I can do that."

Day, whose current ranking of 16th in the world is his lowest since November of 2013, is also trusting that Williams can help him with his quest.

Williams was on the bag for 13 of Woods' 15 major victories, including his epic 15-shot victory at the US Open at Pebble Beach in 2000.

"To have someone like Steve on the bag is very instrumental in my career with regards to where I want to go and the trajectory I want," Day said. "My goal is to get back to number one in the world."

Day has five top-10 finishes this season, including a tie for fifth at the Masters.

But he missed the cut at the Memorial last month and said he's "very disappointed with how this year has progressed."

At the urging of his wife, Ellie, and his professional team, Day said he would be "more selfish" with his time.

"I need to start working harder, that's plain simple," Day said.

He added that it was "a little bit intimidating" to have Williams on his bag. In addition to working for Woods, Williams was on the bag for Adam Scott when the Aussie won the Masters.

"He's had a lot of great ball-strikers that he's worked for," Day said. "I was excited and intimidated in the same breath."

Day, who won his only major title to date at the 2015 PGA Championship, claimed eight US PGA Tour titles in 2015 and 2016. He didn't win again until 2018, when he claimed two titles, but he has since stalled again and said he thinks Williams can make a difference.

"I think Steve will take me to that next level, and I'm hoping that's the case," Day said.