"I did a lot of work in the two weeks that I had off and it showed nearly instantly and I'm already playing some good stuff".
"Sometimes I get in my own bubble", Mickelson said.
Casey finished with a birdie that was worth US$152,000 (NZ$225,000) because he wound up alone in second place. Casey and FedEx executive Don Colleran had a one-shot lead in the pro-am.
The Portrush man turns 39 in July and while he lost his PGA Tour card a year ago and must rely in invitations to fill out his schedule, he wants desperately to be at Royal Portrush for The Open.
Undoubtedly Mickelson will be the subject of considerable conjecture when the golf world returns to Pebble Beach in June for the 119th playing of the U.S. Open, the major that defines Lefty's bouts of futility more than anything.
Mickelson began by calmly knocking his tee shot on the par-three 17th to inside 10 feet and Casey followed suit, but when the 41-year-old failed to sink an eight-footer for birdie his hopes of snatching the win were effectively ended. Casey could barely see the 3 feet of grass between his ball and the cup on the 16th hole and said there was no way to complete two holes in six minutes.
"We've got two tough holes left and I don't want to jump the gun and get past that", Mickelson said. Mickelson poured it on, showing his skills have not deteriorated a bit at age 48, controlling spin beautifully to back pin positions. This time, he closed with a 71 and lost to Mickelson's 65, which matched the low score of the final round.
Casey called Mickelson's effort "a phenomenal round of golf" but added: "It's not over yet".
Phil Mickelson was not happy at having to return on Tuesday to play the final two holes after bad light stopped play. Six runner-up finishes, some in the most heartbreaking fashion, dot his career, the tournament keeping him from a career Grand Slam. He finished three shots behind Graeme McDowell in 2010, the last US Open at Pebble.
His first one also was a Monday finish, when players returned in August - more than six months later - to finish the third round of the weather-plagued tournament to make it official.
And while the fairway lines already have been cut much tighter at Pebble, the conditions were so soft that balls plugged in the fairway when they landed and greens easily held shots even from the rough.
His path to victory these days is never going to be mundane, even in a tournament where he went an entire round without missing a fairway - a 20-year first.
"That's probably a good omen", Mickelson said.