Only 10 players in the 30-man field could complete play before darkness fell at the East Lake Country Club, and Clark was one of them as he fired an 8-under 62.
Play was halted by thunderstorms for four hours and more bad weather was in the forecast for Friday, causing PGA Tour officials to move up the tee times for the second round.
Woods -- trying to become the inaugural winner of the FedEx Cup and its $10 million first prize -- stood at 4-under through 11 holes when play was halted. Steve Stricker and Phil Mickelson, either of whom could win the Cup with a victory in this tournament, fell back early.
Mickelson rallied from an early bogey and double bogey to get to 1-under through 12. Stricker was 1-over through 11.
Padraig Harrington was in second place with a 63 while John Rollins was alone in third with a 6-under 64. Mark Calcavecchia was also at 6-under through 16, while both Hunter Mahan and Woody Austin were 5-under through 17.