A morning fog delayed the start of the round for 2 hours and 54 players were still on the CordeValle Golf Club course when darkness forced a halt to the action.
Casey, with 11 international victories to his credit but only a one-time winner on the PGA Tour, shot a 7-under 134 to reach the halfway point of the event at 8-under 134.
Three-time major championship winner Ernie Els, seeking his first victory in more than a year and his 19th on the tour, had moved into a tie for second at 7-under when play came to an end.
Els had two holes left when darkness fell and shared second place at 7-under with Bud Cauley, who shot a 66 Friday.
Woods will play on the weekend at a full-field event for the first time since the Masters, where he suffered knee and ankle injuries that almost wiped out his entire season.
Since then he has withdrawn after nine holes at The Players Championship, finished tied for 37th at the limited-field WGC-Bridgestone Invitational and missed the cut at the PGA Championship.
Woods, who has not won in two years and has fallen out of the Top 50 in the world rankings, shot a 68 Friday for a 36-hole total of 1-under 141. The cut was expected to fall at either even par or 1-over. He was tied for 40th when play was stopped.