Graeme McDowell took sole possession of second place after the second round of the Chevron World Challenge in Thousand Oaks, California, but Tiger Woods extended his lead to four shots.
Woods, whose two-shot lead at the end of the first round saw him top a leaderboard for the first time since winning the Australian Masters a year ago, consolidated his position with a round of six-under 66 to put him 13 under overall.
McDowell carded a 69 as fellow Northern Irishman Rory McIlroy could only manage a 70, his scorecard blotted by a double-bogey on the 18th.
Luke Donald, who began the day in a tie for sixth, fired a round of 66 to move level with McIlroy on eight under.
But the big mover on the day was Paul Casey, who had a disappointing 73 on Friday but bounced back with a 65 - highlighted by a stunning hole in one on the 12th - to catapult himself from 14th to fifth.