Tiger Woods carded a seven-under-par 65 to open a one-stroke lead midway through the Target World Challenge.
The defending champion completed 36 holes at 11-under 133, one shot ahead of Padraig Harrington and Davis Love III.
Woods had begun the day tied for eighth in the limited field of 16 at Thousand Oaks in California, four shots behind leader Jim Furyk, who opened with a tournament record of 64.
But Furyk struggled in the second round with a two-over 74 and dropped back to eighth, level with Colin Montgomerie after he added a 70 to his opening 68.
Furyk tied Phil Mickelson for the day's second-worst score, ahead of only USPGA Championship winner Rich Beem, who posted a five-over 77 and is last at seven-over 151.
South African Nick Price fired a two-under 70 and is alone in fourth at 135.
Germany's Bernhard Langer, recently inducted into the World Golf Hall of Fame, matched Woods for the low score of the day and shares fifth with South Africa's Retief Goosen and David Toms at 137.
USPGA Tour Championship winner Vijay Singh rebounded from an opening 75 to shoot a three-under 69 and is 14th at the halfway point.