FIRST ROUND LEADERBOARD:
-4 I Poulter (Eng), Z Johnson (US) -3 P Harrington (Ire) -2 J Furyk (US), YE Yang (Kor), S Cink (US) Selected others: -1 L Westwood (Eng), G McDowell (NI) +3 P Casey (Eng)
Poulter went into the tournament as the world number 14
England's Ian Poulter is joint leader after the first round of the Chevron World Challenge in California after an opening four-under-par 68.
Poulter double-bogeyed the 18th to fall back into a tie for first with American Zach Johnson. The pair lead Ireland's Padraig Harrington (69) by one.
Lee Westwood and Graeme McDowell carded 71s, while Paul Casey is propping up the field after a three-over 75.
Tiger Woods withdrew after sustaining injuries in a car crash last Friday.
The world number one has hosted the tournament since 2000, and is a four-time winner.
The field in Thousand Oaks is comprised of 18 players, with invitations extended to the defending champion, the winners of golf's four majors and two exemptions selected by the Tiger Woods Foundation.
Although the $5.75m (£3.5m) tournament is not an official PGA Tour event, it features 11 of the world's top 20 players and is carrying ranking points for the first time.
Last year's winner Vijay Singh is also injured and he and Woods were replaced in the field by American Justin Leonard (72) and McDowell.
Woods was also unable to compete a year ago, as he recovered from reconstructive surgery on his left knee, though this is the first time in a decade he has not been present at Sherwood to host his own event.
Earlier this week Woods issued a public apology to his family, amid continuing speculation about his private life following a car crash last week.
Woods was found bleeding and semi-conscious after his car hit a tree and fire hydrant outside his Florida home.