Phil Mickelson admits the prospect of achieving the career grand slam weighs on his mind now more than ever.

The world's 17th-ranked golfer has endured a precipitous regression in recent years, but there is still one major championship for which Mickelson is almost certain to contend year over year: the U.S. Open, which begins on Thursday.

As the tournament's ultimate bridesmaid, having finished runner-up a record six times without winning, Mickelson said claiming the tournament remains the last big priority of his professional golfing career.

"I could BS you and tell you I don't think about it; no, I think about it all the time,'' the five-time major winner told a news conference on Wednesday.

"This is the tournament I want to win the most to complete the four majors. There's no question."

Mickelson, who will celebrate his 46th birthday as the first round opens, is no longer a spring chicken.

And though the U.S. Open historically is one of his best tournaments, his last two attempts have finished tied for 28th and 64th.

He also missed the cut at the Masters in April and has not won an event since 2013.

Should he at long last conquer the U.S. Open, Mickelson would join Gene Sarazen, Ben Hogan, Gary Player, Jack Nicklaus and Tiger Woods as the only players with all four modern major championships to their credit.

And he would become the fourth-oldest to win a major.

"I have to put that out of my head and try to execute and be patient and not think about results," Mickelson said. "You start thinking about results, you'll never play your best golf.

"So I have to put that in the back of my head, but there's no question that starting this year and every year forward until I ultimately win this tournament, it will be my biggest thought, my biggest focus because I view those players who have won the four majors totally different than I view all the others.''