The big left-hander flew home to San Diego to attend his daughter Amanda's eighth-grade graduation, then landed back in Pennsylvania at 3:30 a.m., less than four hours before his tee time.
He posted three birdies over his last 10 holes for a three-under-par 67 during a weather-interrupted opening round that marked his lowest first-round score in 23 US Opens.
A four-time Major winner, Mickelson is seeking his first US Open triumph after a record five runner-up finishes, but said he did not want to miss her graduation ceremony.
"I told her that I want to be there. I don't want to miss that. I don't want to miss her speech. I don't want to miss her graduation," Mickelson said. "She's worked very hard and I'm very proud of her."
Mickelson said the ceremony was at 6 p.m. and that he boarded his plane back to Merion Golf Club at 8 p.m and managed to get just enough sleep.
"A couple hours on the plane, an hour before we teed off and then an hour during the (weather) break," he said. "I feel great."
Mickelson said he used his travel time wisely.
"I was able to take the time on the plane to read my notes, study, relive the golf course, go through how I was going to play each hole, where the pins were, where I want to miss it, where I want to be, study the green charts," he said.
"It gave me a great few hours to study my notes and get mentally prepared."
Mickelson got two days of preparation work at Merion last week before heading to the St. Jude Classic in Memphis, where he tied for second place.
The American was going to put more practice time in at Merion, but heavy rain at the classic layout on Monday convinced him he would be better off going home early for the graduation and get in some solid work on the range and greens in California.
"What I needed was to get my game sharp, to get my touch sharp. And having a nice practice facility and nice weather for the last couple of days allowed me to do that," he said. "So it worked out great on both ends."
After balancing a bogey with a birdie over his first eight holes starting from the 11th on Thursday, Mickelson made his move during the second part of his round.
He sank a curling 25-foot birdie putt at the first hole, a short birdie putt at the seventh and then drained a 20-footer for birdie for the outright lead at the par-three ninth.
Mickelson has come painfully close in some of his runner-up finishes at the Open.
His first near miss came at the 1999 US Open at Pinehurst, when Amanda was due to be born any day, and where he lost a tense duel with Payne Stewart on the 72nd hole.
Mickelson had said he was prepared to leave Pinehurst the moment he got a message that his wife, Amy, was giving birth.
After sinking his winning putt, Stewart gave Mickelson an emotional embrace and whispered in his ear that he was going to be "a wonderful father."
"If I'm able to ultimately win a US Open, I would say that it's great, because I will have had let's say a win and five seconds," Mickelson said about his relationship with the national championship.
"But if I never get that win, then it would be a bit heart breaking."
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