With heavy rain forecast for later in the day, play started early at Crooked Stick Golf Club and Singh went right to work mixing seven birdies with a single bogey for a two day total of 13-under 131 in the penultimate FedExCup play-off event.
"I've got to keep it going," Singh, who has not won on the US PGA Tour since 2008, said. "I've been playing well for two days for a while now, but I need four days of good playing.
"Sooner or later I think four days is going to happen, and hopefully it starts this week."
The smooth swinging Fijian, who started his day one shot off the lead, began on the back nine and was quickly on the move with birdies at the 11th and 15th before grabbing the lead with four consecutive birdies from the 18th.
His only bogey came at the par-four fourth but then signed off with another birdie at the last to secure a one shot lead over Woods (67), McIlroy (68) and American Ryan Moore (66).
A triple Major winner who will turn 50 in February, Singh has won just about everything there is to win in golf including the 2008 FedExCup and the $10 million bonus.
Briton Lee Westwood shot into contention with the joint best round of the day, a seven-under 65 that featured six birdies, an eagle and one bogey to join American Bo Van Pelt (69) two back of the leader at 11-under.
A day after putting on a dazzling display of golf, Woods and McIlroy were the main attraction again on Friday but were unable to produce the same magic.
Playing together in the same threesome, Woods and McIlroy, the only players to win three US PGA Tour titles this season, had spent Thursday trying to top each other but on Friday they were more concerned with damage control.
Woods turned in a bogey-free effort but was unable to build any momentum until late in this round when the 14-time Major winner birdied three of the final four holes.
"I fought hard, I didn't have much today," said Woods. "Swing wasn't quite there and I was just grinding along and just trying to get to double digits under par.
"That was the goal, the way I was hitting it. I was just slapping it everywhere and somehow just grind away and find a way to shoot something where I was in double digits by the end of the day."
Mcllroy, who shared the overnight lead with three other players, had birdies on his opening two holes but the 23-year-old Northern Irishman was in for a more adventurous afternoon as he offset six birdies and an eagle with four bogeys.
"It wasn't as good a day as yesterday," admitted McIlroy. "I didn't hit the ball quite as well, but scored well and did enough to go out there and shoot another good number and set myself up in a good position going into the weekend."
There will be no cut at the BMW Championship with all 70 golfers in the elite field vying for one of 30 spots in the September 20-23 Tour Championship finale in Atlanta where the overall points winner gets a $10 million bonus.
There were also a few unfamiliar names on the leaderboard, South Korea's Noh Seung-yul (66) and Canadian Graham DeLaet (70) boosting their chances of earning a ticket to East Lake Golf Club lurking just three off the lead at 10-under 134.