Play was delayed for three hours.
Casey called Mickelson's effort "a phenomenal round of golf" but added: "It's not over yet". Casey had great par saves, and then Mickelson took off. Mickelson was on the 17th tee, shaking his head when he heard the siren to stop play.
With two delays approaching three hours, it appeared certain the final round would not be completed Sunday.
"We've got two tough holes left and I don't want to jump the gun and get past that", Mickelson said. "I wish we could do it tonight". The greens were beat up (on Sunday). We had a chance to come out today on fresh greens, better weather and being able to see a lot better.
I've received stick for years for backing Mickelson repeatedly in tournaments he likes but it's paid off yet again and I've already backed him for this week's Genesis Open too.
The victory, Mickelson's first since the WGC-Mexico Championship last March and first on USA soil in six years, carries special significance for him. 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.
"The ball striking was very good but I didn't putt well at all this week".
"When guys get in their 40s, two things decline, their putting and their swing speed", said the new world number 17, who is now swinging at close to 120 miles per hour.
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It would be wonderful for Phil to claim the career grand slam at Pebble with a second win there this season. To have Mickelson complete the Grand Slam at the course that means so much to him, and where he's had so much success, would be a fitting cap on his career.
Mickelson has come close, with six runner-up finishes at the U.S. Open.
Phil Mickelson was not happy at having to return on Tuesday to play the final two holes after bad light stopped play.
Now a victor at that level, his tour card is safe for two years, which will give him a completely different outlook. "[It means] absolutely nothing", he said.
There was never a reasonable chance to finish in his pro-am format, with mostly foursomes across the golf course.
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.