"I thanked Paul this morning because I sometimes in my own little bubble I don't see the big picture", Mickelson said.
Casey also closed Monday's play with a birdie putt at 18 to finish alone in second at 16 under.
"And so it's gratifying to see the results and to finish it off the way I did".
Mickelson's first Pebble Beach title back in 1998 came when the weather was so bad that the final round was actually played on a Monday in August - six months after the tournament began.
Hale Irwin, who won the 1990 edition of the US Open at 45 years and 15 days old, is the oldest US Open victor to date so the stats are really against Phil Mickelson, who turns 49 on the Sunday of this year's edition.
It was a decision Mickelson clearly disagreed with, visibly shaking his head when the horn blew to officially halt play.
"The challenge is getting myself to play my best".
The last two groups didn't tee off until after two weather delays, with rain postponing the start of play for an hour and a hailstorm bringing another two-hour hold-up.
The final round of the AT&T Pebble Beach Pro-Am was delayed Sunday by the kind of weather that once gave this event its reputation, including a hail storm that covered the greens in white. "I know a lot can happen in these two holes and they have happened in the past, so I want to stay focused and just come out tomorrow and try to finish it off". But he said he understood Casey's perspective.
"I get exactly where Paul is coming from", Mickelson said.
He gained ground with birdies at the par-five second and the fourth, then made back-to-back birdies at the ninth and 10th - two of the toughest holes on the course. As darkness descended on the course, Mickelson had just tapped in on the 16th green with Casey still to putt.
His brilliant play still shared the stage with weather that was freakish even by Pebble standards.
He will return to Pebble Beach in June for the US Open, where he made his pro debut in 1992.
Day has picked up two shots through 14 holes of his final round to climb to 11 under, sharing sixth place but five shots behind Casey. "I'll deal with that in six months".
Asked in a greenside interview what the performance did for his confidence, Mickelson said: "Absolutely nothing, because it's nothing like the course we'll see".
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.
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