Zach Johnson has finished one shot ahead of Matt Kuchar at the end of the first round of the Northwestern Mutual World Challenge at Sherwood Country Club.
The 37-year-old American carded a five-under 67 in Thousand Oaks, California, replete with seven birdies and two bogeys.
His compatriot and former Ryder Cup teammate Kuchar shot five birdies during his round, with the highlight being an eagle three at the 11th.
However, dropped shots at seven, eight and 17 mean that he is four-under heading into the second round.
Hunter Mahan and Bubba Watson share third place on two-under-par, while Tiger Woods, who is hosting this week's event, is one-under.
Northern Irish duo Graeme McDowell (E) and Rory McIlroy (+1) are the only players not from America to feature in the top 10 in the 18-man field.
Lee Westwood (+2) and Ian Poulter (+4) are also guests of the 14-time Major champion.
Bottom of the field is rookie Jordan Spieth (+5).
