Johnson holds early lead
Zach Johnson finished day one of the Northwestern Mutual World Challenge five-under-par to lead by one shot in California.
Johnson hit a purple patch with five birdies in seven holes from the 10th to finish the day one clear of Matt Kuchar, with only five of the 18-strong field under par for the day.
Johnson made a tremendous start at the Sherwood Country Club with birdies in his opening two holes and although he bogeyed the par-four seventh, he found a tremendous patch of form to ensure even a dropped shot at the last did not keep him off the top of the leaderboard.
Kuchar's round was highlighted by an eagle at the par-five 11th, but he was hamstrung by three bogeys.
The other three men under par were also Americans - Hunter Mahan and Bubba Watson at two under and, in his own tournament, Tiger Woods at one under.
"I felt like I hit the ball decent today," said Woods. "I missed two short ones (putts) there at one and the kick-in at 18.
"That's a couple of shots right there, and I'm only two back. I had a couple of good ones (shots) on the back nine today that ended up in some very interesting spots. The golf course is kind of set up that way right now.
"There are some tough pins out there. You miss them on the wrong spots, you're going to pay a price and I think that's kind of what's signified in the scoring. No one is running away with it. Nobody went low today."
Reigning champion Graeme McDowell was the leading overseas challenger at level par, alongside Jim Furyk and one clear of fellow Northern Irishman Rory McIlroy.
McDowell is looking to continue his love affair with a tournament he has won twice and finished runner-up once in his only three appearances to date.
However, while McDowell matched Johnson's dream start, he was in rather more erratic form on Thursday with four birdies and four bogeys leaving him even for the day.
"Generally I was quite happy the way I hit the ball," said McDowell.
"I hit a few squiffy ones, but there's room for improvement. Get back out there tomorrow and there are lots of birdie opportunities on this course.
"I think that's why I've always enjoyed it, why I've always felt comfortable on it. It offers you a lot of chances if you play decent."
It was not a great day for England's two entrants: Lee Westwood was two over par and Ian Poulter four over.