Michael Campbell struggled to a level-par 72 in the second round of the Target World Challenge in California but still increased his lead.
The New Zealander reached the halfway stage at nine under on 133 – three clear of Padraig Harrington and Darren Clarke.
Campbell, who fired a course record-tying 63 on Thursday, had moved five ahead of Kenny Perry and Clarke after 12 holes but then dropped four shots – including a double-bogey on the last – to give the rest of the 16-man field some hope.
Campbell’s stumble has left him catchable entering the weekend, with Harrington carding a 70 and Clarke a one-over 73.
Tournament host Tiger Woods is back in 14th place following back-to-back rounds of 72 but has never finished worse than first or second in this event since 1999.
“I just had a hard time finding my lines, a hard time seeing the putts, seeing the lines, feeling the breaks,” Woods said.
“I’m having a hard time feeling the slopes in my feet, in my hands, my body. I’m just having a hard time with it.”
Kenny Perry (70) and Fred Couples (68) are tied for fourth at five under on 139 with England duo Luke Donald and David Howell alongside Chris DiMarco a shot further back after each shot 68s.
Scotland’s Colin Montgomerie is in last place after a two-under 70 left him on 146.