Mickelson is on the verge of winning for the 44th time in his career, and matching Mark O'Meara with five victories at a tournament he first played in 1995.
"I get exactly where Paul is coming from; I have pretty good vision, so I could see fine and I am playing well".
"I thanked Paul this morning because I sometimes in my own little bubble I don't see the big picture", Mickelson said. "This golf course, it looks very gettable [but] it can expose you".
This was the fourth time that Casey had a 54-hole lead of at least two shots on the PGA Tour and failed to win.
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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".
"So at the end of previous year, even though I played poorly, I had something happen where it seemed like overnight - it had really been in the works - where my driver speed shot up 5, 6 miles an hour, which rarely ever happens to anybody, let alone somebody in their late 40s", he said.
That also led him to enter this week's Genesis Open, which he had not planned to play, prior to defending a title at the WGC-Mexico Championship next week.
Casey called Mickelson's effort "a phenomenal round of golf" but added: "It's not over yet".
Play has just resumed at the final round of the US PGA Tour's Pebble Beach Pro Am after being suspended for just over two hours due to a hail storm.
The four-time Pebble Beach victor nearly holed out from the ninth fairway to leave a tap-in birdie and complete a front-nine 33, before rolling in from 12 feet at the next to move into a share of the lead. 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. "In all honesty, it's a good thing to play the last two holes in fresh conditions".
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.