Compatriot Paul Casey birdied the final two holes at Pebble Beach to return a three under opening round.
Also contesting the event played across US Open rotational venue Pebble Beach Golf Links, Spyglass Hill and Monterey Peninsula Country Club are Australians Aaron Baddeley, Rod Pampling John Senden, Matt Jones, Curtis Luck, Ryan Ruffels and Cameron Davis.
Phil Mickelson fired a 6-under 65 in Round 1 of the Pebble Beach Pro-Am on Thursday, and he hit every fairway for the first time in over 20 years.
Brian Gay and Scott Langley are tied atop the AT&T Pebble Beach Pro-Am leaderboard after round one, but Phil Mickelson stole the show on the fairway. He played it off the beach back to the fairway, hit 7-iron to 10 feet and made the putt for a 69. "I'm not going to chase them as hard as I tried to maybe force or chase them past year because you can get hurt doing that".More news: Volunteers Ready US Aid Set for Venezuela as Maduro Digs In
"I think that one of the rule changes that came about where you can fix some areas of the green that were caused by other players made a big difference in this tournament for everybody, because around the hole you can smooth out some of the spike marks and indentations".
"I was starting to get very frustrated, so let's just say today I was very, very happy to be back on the golf course as it felt good to be out there".
Mickelson is regarded as having arguably the greatest short game in golfing history but has perennially struggled with the driver.
His round included a birdie on the par-three ninth, his 18th hole of the day, to leave him alongside Gay on seven under.More news: Taliban not seeking to seize whole of Afghanistan
Now, the 31-year-old has given himself a hot start to the Pebble Beach Pro Am - where he has recorded five top-six results, including a runner-up last year, without lifting the trophy.
Day, Johnson and Spieth are all part of the world number one club.
"I'm pleased with it (but) I definitely left some shots out there", said Johnson, four days removed from victory at the European Tour's Saudi International.
Teeing off from the 10th, Day mixed five birdies with a bogey to make the turn at four under. "The putts went in instead of missing", he said.More news: Kane is recovering well, says Tottenham boss Pochettino