The Venezuelan birdied four of his first five holes to set the tone for his round and picked up another three shots after the turn, with just a solitary bogey on the par-five 15th.
Just one shot behind is Charles Howell III, who shot 66 on the CordeValle Golf Club, while American compatriot John Mallinger is on four under after 16 holes.
Jeff Maggert, with just one hole left to play on Thursday, is one of seven players in a tie for fourth on three under par.
The American, who has previously represented the United States in the Ryder Cup on three occasions, went in at the turn on one under and picked up back-to-back birdies on the 15th and 16th.
Davis Love III is on two under after 17 holes and Vijay Singh, Ricky Barnes and Robert Karlsson are among those a further stroke behind with Open champion Ernie Els going round in a level par 71.