Rory McIlroy and Tiger Woods are hot on his heels heading into the weekend at Crooked Stick in Carmel, Indiana.
Singh, looking for his first PGA Tour win in four years, followed up his opening 65 by collecting seven birdies on Friday to advance to 13 under par. The Fijian's only dropped shot, after starting on the 10th, came at the par-four fourth.
World number one McIlroy, one of a quartet atop the leaderboard after the first round, remained very much in contention, however. The Northern Irishman posted a topsy-turvy 68, and Woods carded 67, to leave the pair tied for second - a stroke off the pace - alongside Woods' fellow American Ryan Moore (66).
McIlroy, fresh from his victory at the Deutsche Bank Championship in Norton on Monday, picked up six birdies in his second round and eagled the par-three ninth, but four bogeys cost the USPGA champion top spot.
Woods had a steady start to his bogey-free second round but finished with a flourish, surging into contention with three birdies over his final four holes.
England's Lee Westwood and Indiana-born Bo Van Pelt were two shots adrift of Singh after rounds of 65 and 69, respectively.
Singh was hopeful he could continue his fine form into the weekend.
The 49-year-old said: "I've got to keep it going. 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."
Reflecting on his 68 after his opening 64, McIlroy said: "It wasn't as good a day as yesterday [Thursday]. 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.
"I could have hit the ball a little better from tee to green, but I still got it around in four under, so pretty happy.
"I put myself in a great position going into the weekend."
The 23-year-old, leader of the FedEx Cup standings, added: "I just need to try to find a little more consistency. It wasn't that bad out there, but just a couple of missed tee shots and a couple of missed iron shots."
Like McIlroy, his playing partner Woods also acknowledged Friday had not gone quite as well as Thursday, but seemed content with his midway position of 12 under par.
"Today [Friday] was a struggle," the 36-year-old American said. "I fought hard to shoot a number.
"I fought hard. I didn't have much today [Friday]. The swing wasn't quite there, and I was just grinding along here 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, and I just happened to get a couple more."