Overnight leader Casey had a three-shot advantage over Mickelson at the start of the day, but the five-time major victor made up the deficit with three birdies in the front nine and three more after the turn.
He started the day three shots off Casey's lead but roared to the top of the leaderboard with six birdies that put him 18-under for the tournament with two holes remaining.
"I just think that both myself and Tiger are going to have a really, really good year this year", Mickelson said.
Lost in the late start - one hour delay from rain, a two-hour suspension from hail - was sheer brilliance from the 48-year-old Mickelson who didn't come remotely close to making a bogey and was on the brink of a fifth victory in the AT&T Pebble Beach Pro-Am.
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"It's been a great day so far (but) it's not over, we've got to finish it off", Mickelson said. "This golf course, it looks very gettable [but] it can expose you".
Mickelson recorded his 44th PGA Tour triumph in style with a closing seven-under-par 65 at Pebble Beach.
Casey said he didn't see why they should continue in the gathering darkness when they wouldn't be able to complete the round. Play was suspended on February 2 and resumed and ended on August 17.
The Portrush man turns 39 in July and while he lost his PGA Tour card previous year and must rely in invitations to fill out his schedule, he wants desperately to be at Royal Portrush for The Open.
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"From my angle it was are we going to get 18 done", he said.
Casey called Mickelson's effort "a phenomenal round of golf" but added: "It's not over yet".
Casey blinked first with a bogey on the 11th hole, and another on the par-3 12th when his tee shot came up short and into the bunker. It's where his grandfather, Al Santos, was among the first caddies when the course opened in 1919, and Mickelson still uses as a marker the 1900 silver dollar his grandfather kept in his pocket to remind him he was not poor. A Lefty US Open win in June would be the sporting story of the year for me and just about the only thing that could beat it would be him winning it next year, back at Winged Foot, at the age of 50.
There is nothing better in golf than eagling the last hole and that fact will not be lost on David Law after the young Scotsman wrapped up his first European Tour title in style on Sunday.
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American Scott Stallings holds the clubhouse lead on 15 under after his six-under par 66, with Jason Day finished on 13 under, level with South Korea's Kim Si-woo.