Zach Johnson finished day one of the Northwestern Mutual World Challenge five-under-par to lead by one shot in California.

Reigning champion Graeme McDowell was the leading overseas challenger at level par, alongside Jim Furyk and one clear of fellow Ulsterman Rory McIlroy.

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 hot streak of form to ensure even a dropped shot at the last did not keep him off the top of the leaderboard.

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.

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.

It was not a great day for England's two entrants: Lee Westwood was two over par and Ian Poulter four over.