Rain and lightning forced a 3-hour delay early in the day and almost half the field was still competing when darkness fell.
Those who had finished their rounds at 67 included Dustin Johnson, Jay Williamson and Marc Leishman. Tommy Armour III and Nathan Green were at 3-under through 16 holes.
Day said he almost withdrew early Thursday due to illness.
"I'm on these heavy antibiotics that make me very sick," he said, "and I was feeling pretty sick this morning. I really didn't think I was going to play. Warming up, I got out there and felt okay and then that rain delay helped a lot. I just kind of sat down and rested, drinking some water, and just trying to rest up a little."
Defending champion Rory Sabbatini shot a 68.