Due to a 90-minute fog delay at the start of the round, Immelman did not tee off until after 3 p.m. local time. He was in the last group, holing a 15-footer in the dark for his fourth birdie in six holes, a 6-under-par 65 par and an aggregate of 11-under 131.
"It was great to hole that one in the dark after all the ones I missed earlier," Immelman said.
The 22-year-old South African, a former British Amateur championship runner-up and South African Amateur winner, has been second three times on the European Tour this season.
Twenty-nine of the 120 golfers failed to complete their rounds.
Harrington, hoping to become the first Irish winner of the European money title, followed up his opening 65 with a 66. The Dubliner, who collected seven birdies with two bogeys, sits third behind South Africans Retief Goosen and Ernie Els in the race to finish the season as Europe's No. 1 player.
Fellow Irishman Des Smyth, the oldest winner in the history of the European Tour international schedule, matched the 11-year-old course record of 63 to move into a third-place tie with Adam Scott of Australia and Maarten Lafeber of the Netherlands, one stroke off the pace.
Twice a winner on this course, Harrington finished the front nine in 31 and was three clear of the field and eight ahead of Goosen. But the South African, who began his round at the 10th, played the front nine in 31 for a 69, while Harrington bogeyed two of his last five holes.
"The bogey at the last came because I feel asleep," Harrington said. "I was thinking about lunch or something. I thought my day's work was over and it wasn't."
Goosen, who is trying to retain his Order of Merit crown, was tied for 13th at 7-under 135.
"I am not looking at what Retief was doing," Harrington said. "It's hard enough to control what I am doing. But overall I'm happy. I'm leading and you are always going to hit some bad shots."
Smyth, who in March 2001 claimed the Madeira Island Open, birdied seven of his last nine holes to come home in 29. The 49-year-old had nine birdies overall to produce the second-lowest round of his 29-year-old career.
"It's very exciting," Smyth said. "I love just breaking 70 these days. I always think I've had a successful day if I do that."
He chalked up his performance to the fact that he is playing the the U.S. Senior Tour qualifying school in two weeks.
"I've got to go through regional qualifying first and I think there's an edge coming back into my game," Smyth said. "There's only eight cards on offer, and it's a big challenge. I will have to play my best to get through it all."
Smyth's 1981 Ryder Cup teammate and fellow 49-year-old, Sam Torrance, shot a 66 to put himself among a group two shots in back of the leaders.
Torrance, the captain of Europe's winning Ryder Cup squad this year, birdied his last three holes to join Miles Tunnicliff of Britain and Swedes Marten Olander and Fredrik Jacobson in pushing England's Lee Westwood from first to sixth place.
"I'm certainly relaxed and I think we're all on a high," said Torrance. "I hope it will last forever."
Westwood turned in a 70 after his opening 64.