Singh, who has surpassed Tiger Woods as the world's No. 1 player, is seeking his eighth victory of the year.
Kevin Na was also at 5-under through 14 holes when play was called for the day. Fog delayed the start of the first round, which will be completed early Friday.
Singh, who has wrapped up Player of the Year honors with his big season, will break Woods' single-season earnings record if he wins this week.
"I do the same thing over and over again with my golf swing," Singh said. "The more I do it, the better it gets. I think now my swing is into a plane where if I just keep doing what I'm doing, it should stay the same."