On his back nine, the 43-time PGA Tour champion tallied five birdies at Monterey Peninsula Country Club.
Keith Mitchell hit a left-handed shot from a wooden bridge at Monterey Peninsula through the rough and onto the green that led to par and duly impressed his amateur partner, PGA Tour commissioner Jay Monahan. "I have a lot of confidence in my game right now".
The evergreen 48-year-old had a quiet front-nine before enjoying a run of three birdies in four holes after the turn, and he picked up further shots at 16 and 18 to ensure he would end the day just a stroke adrift of early pace-setters Brian Gay and Scott Langley.
Day carded seven birdies in his 65, while two-time champion Dustin Johnson and 2017 victor Jordan Spieth also took advantage of the favourable scoring conditions on the Monterey Peninsula course as they opened with 66s, while last week's Phoenix Open runner-up Branden Grace returned an impressive five-under 67 at Pebble Beach.
The AT&T Pebble Beach Pro-Am returns to the world famous Pebble Beach Golf Links on the Monterey Peninsula in California.
"I may have done it before, but I don't ever recall doing it", a satisfied Mickelson said afterwards. Spieth chipped in for birdie and made plenty others to join Johnson at 66.
Defending champion Ted Potter Jr. struggled to an 82 at Spyglass Hill and is 155th, or dead last in the field.
"It's about how do I improve to get myself into contention this week and then what do I do next week and just staying very present and recognising the longevity of a career", Spieth said.
And now the real work begins.
And a few more photos of the sports power couple. The starting times were moved up an hour on Friday.
"I think it's a big factor-you don't know how it's all going to play out, but I think that it's important to get off to a good start", he said. Told there was little chance, Johnson shrugged.
"I guess it's just lack of sharpness, not having played for seven weeks".
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