SECOND ROUND LEADERBOARD:
-9 YE Yang (Kor) -7 K Perry (US), P Harrington (Ire), I Poulter (Eng) -6 S O'Hair (US), L Westwood (Eng), Z Johnson (US) Selected others:-4 G McDowell (NI) Level P Casey (Eng)
Poulter had led after the first round but is still handily placed
Joint overnight leader Ian Poulter is two shots off the pace at the Chevron World Challenge after the second round.
Poulter shot a 69 to lie seven under alongside the Republic of Ireland's Padraig Harrington, who had a 68, and American Kenny Perry, who shot a 65.
They trail USPGA champion YE Yang by two shots after he equalled Perry with a 65 at Thousand Oaks in California.
Lee Westwood is on six under after a 67 while Graeme McDowell is four under after carding a 69 on Friday.
South Korean Yang, who shot to prominence when he out-duelled Tiger Woods to win the USPGA in August, was fighting illness during his round.
"I'm actually coming down with the flu or a fever," said Yang, who arrived from China on Monday and said he was starting to feel ill then.
"I've got a pounding headache. I didn't try to force anything."
The tournament is hosted by world number one Woods but he is not present after suffering injuries in a car crash last Friday.
"It's disappointing because it's Tiger's event - competing with him is always fun," said Yang.
The field in Thousand Oaks is made up 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.
Earlier this week Woods issued a public apology to his family, amid continuing speculation about his private life following his car crash.
Woods was found bleeding and semi-conscious after his car hit a tree and fire hydrant outside his Florida home.