It would be wonderful for Phil to claim the career grand slam at Pebble with a second win there this season.
"And so it's gratifying to see the results and to finish it off the way I did".
'I don't want to be too emotional because I don't want to rub him the wrong way.
"All I'm focused on is the Masters. I wish we could do it tonight".
Mickelson's longest putt for par was 4 feet.
"Paul made the tough call, but it was the right decision in that he protected himself and myself for the competition, " Mickelson said. "And I have believed for some time that if I play at my best, it will be good enough to win tournaments". "Phil has put together a spectacular round of golf so far - 6 under, no dropped shots".
"That's one of the worst 3-footers I've had to hit", Casey said.
Casey has a four-foot putt to save par on the 16th green when play resumes at 8am (4pm GMT) on Monday, while Stallings holds the clubhouse lead on 15 under after a six-under 66.
He also won the Pro-Am with Don Colleran, the chief sales officer for FedEx. Mickelson was on the 17th tee, shaking his head when he heard the siren to stop play.
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 won on American soil for the first time since the Phoenix Open in 2013.
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.
His brilliant play still shared the stage with weather that was freakish even by Pebble standards.
Temperatures had been in the mid-40s, but wind chill made it feel like the upper 30s.
Sam Saunders, whose grandfather Arnold Palmer was among the Pebble Beach owners, scooped up hail and tossed it like a snowball. Patrick Reed's brother laid on his back and tried to make a snow angel. They were waiting to tee off when sunshine gave way to a hail storm that pounded the golf course as players ducked under umbrellas.
Casey bogeyed the 11th after failing to get up and down from the rough, giving Mickelson the outright advantage, with the Englishman then missing a six-footer to save par at the next to fall two behind.
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. He finished three shots behind Graeme McDowell in 2010, the last US Open at Pebble. And it is where he will turn 49 on the final day of the 2019 U.S. Open.
'Sometimes I get in my own bubble, ' Mickelson said. "We had a chance to come out today on fresh greens, better weather and be able to see a lot better". I have good vision, I can see fine and I wanted to continue.
Some players finished in the dark with no chance of winning, but showed the effect of playing without light.