A weather delay pushed play back and darkness fell before Sim finished. Duke is in the clubhouse at 12 under and is tied with Sim, who has two holes to play.
Duke shot a 7-under 65 Sunday -- the tournament's low round. He had four birdies and an eagle on the front nine with one bogey, for a 5-under 31.
Sim was 1 under on the day, with two birdies and a bogey.
Play will resume at 8 a.m. Monday.
Jarrod Lyle was two strokes back at 10 under and seven players were tied at 9 under.
Weather delays have kept the tournament at The Daniel Island Club behind schedule.