Dustin Johnson made a fine start to his bid to end the weekend as world number one by firing a first-round 66 at the Genesis Open.

Padraig Harrington isn’t in the race to top the rankings, but he too had a good opening round and his four under 67 leaves him with a share of tied seventh, three off the lead held by American Sam Saunders.

Johnson reached the clubhouse on five-under par, two shots off the lead, as he chases a victory that could see him leap from third to first in the rankings.

For that to happen, current number one Jason Day has to finish worse than a three-way tie for third - and the Australian ended day one down in joint 74th, sitting on level par through 16 holes as darkness suspended play early in California.

Japan's Hideki Matsuyama is also in with a shout of being world number one on Sunday evening, but needs to win while Day finishes lower than 24th.

And Matsuyama - who was in the same group as Day for the opening round - lies one-under par after 16 holes, six shots adrift of leader Saunders.

Shane Lorwy looks like he’ll struggle to make the weekend cut, having shot three over through 16 holes on a fog-delayed first round. Graeme McDowell sits on two under having reached the clubhouse in 69.

Johnson, starting on the back nine, birdied the 11th, 14th, 17th and 18th to reach the turn in 32.

He overcame being stung by a bee on the first to gain further shots on the second and third but his only blemish of the day then came on the par-three fourth.

"I missed a pretty easy par putt on the fourth but I played really solid all day long,” he said.

"It would be great to get (number one) but I'm focused on this week. I just want to put myself in a position to have a chance to win on Sunday. That's the only thing I'm really thinking about."

Day struggled at Riviera Country Club, dropping shots at the fourth, eighth, ninth and 12th to cancel out gains on the first, sixth, 11th and 15th.

Harrington has started well in California

Matsuyama birdied the seventh but dropped a shot on the 10th, before ending his day with a gain on the 16th just before play was suspended.

The duo are scheduled to return early on Friday to complete their round after fog caused early delays in Pacific Palisades.

Once the fog lifted, the players enjoyed ideal conditions and Saunders took advantage with a flawless seven-under 64 as he chases a maiden PGA Tour win.

He enjoys a two-shot lead from Johnson, fellow Americans JT Poston, Daniel Summerhays and Brett Stegmaier, and Australia's Cameron Percy.

Harrington is a further shot back after ending his opening round with three successive birdies and he is joined on four under by Phil Mickelson.

England's Luke Donald and world number seven Adam Scott are in a group four shots off the lead.