Jason Day says he'll quit the US Open, even if he's leading, if his wife Ellie goes into premature labour.

Day is in San Francisco preparing for the year's second major championship, starting on Friday (NZ time) at Olympic Club, but his thoughts are also with his wife Ellie back home in Ohio awaiting the birth of their first child due in four weeks.

Runner-up to Rory McIlroy in the US Open last year and rated a decent chance to go one better on Sunday, Day confirmed he was making contingency plans to rush home immediately should the call come.

"We will have planes on hold for an emergency," Day said.

"If I am doing well at the US Open and for some reason she goes into labour - I don't care about the tournament, I have to be there for the birth.

"It might sound stupid to some people to say if you're leading a major you will just leave but family comes first and I'll do that.

"My family will be there my whole life, golf won't.

"Hopefully the timing works out that there aren't any issues."

Englishman Ross Fisher famously made a similar statement when contending in the 2009 British Open. He led early in the final round but finished tied for 13th and thankfully wasn't required to make the mercy dash.

Day comes into the Open in much different circumstances to last season.

Although he finished a distant eight shots behind McIlroy at Congressional, Day's eight-under-par total would have been good enough to win or tie every other US Open bar Tiger Woods' 15-shot victory at Pebble Beach in 2000.

He played his last 45 holes without a bogey, a feat which historians believe to be a US Open record streak for bogey-free holes.

His nine-under-par weekend was the second-best all-time in this major championship.

It was his third consecutive top 10 in majors, his second straight runner-up finish in a major, and his fifth top-10 from the last six starts.

Fast forward to 2012 and Day's year has been punctuated by injury and the off course distraction of the pregnancy.

He has two top-10s but withdrew from the Masters midway through the second round with an ankle injury and has missed the cut in two of his last three starts.

"This year has been a really tough year mentally," Day admitted.

"I have been distracted. And it's hard as I expect so much of myself. I expect to go out and play well every single week and when it doesn't happen I get frustrated.

"I'm not hitting it the greatest. It feels a little cold right now but it's still early in the year and I'm ready to work hard and see how it goes.

"I really want to do well so I hope it all clicks this week."

Adam Scott, Geoff Ogilvy, Aaron Baddeley, John Senden, Brendan Jones, Anthony Summers, Rod Pampling and Alistair Presnell round out the Australian tilt looking for the country's first major in six years.

- AAP