Darren Clarke carded a four-under 69 Friday and stands in third place at 10- under-par 136. Kirk Triplett is one stroke further back at minus-nine, while Scott Hoch stands alone in fifth place at seven-under-par 139. Retief Goosen and Davis Love III are one shot back at minus-six.
Singh got off to a nice start with birdies at the third and fifth to move to seven-under. He dropped his first shot of the tournament when he bogeyed the sixth. After three straight pars, he headed to the back nine at six-under.
'I was a little disappointed with the front nine, didn't birdie nine and also two-putted (for bogey) the sixth hole,' said Singh, a four-time winner last season. 'I just said, 'I'll go play a solid back nine.' I started making some putts, which is always nice on these greens and played solid.'
The Fijian caught fire from there. He birdied the 10th before a par at No. 11. From there, he rocketed up the leaderboard.
'The wind was more favorable today and I hit it right where I aimed,' said Singh of his drive on No. 10. 'I made a good pitch shot and putt there.'
Singh rolled home a 18-footer at the 12th to climb to eight-under. He made it two straight when he converted a three-footer at the next. The 2003 PGA Tour money leader curled in an eight-foot putt at the 14th for his third straight birdie.
He followed with a two-putt birdie at the par-five 15th. When he drained a five-footer at 16, the lead was his at 12-under as Appleby bogeyed the 14th. Appleby briefly tied Singh atop the leaderboard with his birdie on No. 15.
Singh, however, was not done. He continued his hot streak as he drained a putt from over 30 feet out on No. 17 to get to 13-under. He closed out a string of seven straight birdies when he rolled in a short birdie putt at the last.
'I was just playing along,' Singh said. 'I wasn't trying to hard or anything. I didn't make too many mistakes, kept the ball in play and hit it close.'
Singh's stretch of seven straight birdies is one shy of the tour record, which has been done six times previously. His eight-under 29 is two strokes off the tour's nine-hole mark of nine-under 27.
'You've have to make putts over here to score and I didn't do it Thursday as much, but I think the greens paid me back today,' said Singh.
Appleby had a solid round of his own but was overtaken by Singh's hot play. Appleby fired a six-under 67, despite three bogies. He bogeyed the second, but answered with a birdie at the very next hole.
The Australian ran off five straight birdies from the sixth to climb to 12- under and a three-shot lead at the time. However, he bogeyed the 12th and 14th but did manage a birdie in between at No. 13.
Appleby also came back to birdie the 15th to return to 12-under. He climbed within one stroke of Singh with a short birdie putt at the last.
'The wind was playing pretty much the same as Thursday, a little stronger, certainly a lot more of a concern to club selection,' said Appleby. 'There were some birdies to be had out there. But I played pretty well all day. Liked the way I hit it, and liked the way I putted it.'
Tiger Woods and Masters champion Mike Weir each posted rounds of three-under 70 to move to five-under-par 141. That duo is joined in eighth place by Fred Couples, Jim Furyk, John Huston, Peter Jacobsen, Justin Leonard, J.L. Lewis, Shigeki Maruyama and Shaun Micheel.
Ernie Els, the defending champion, carded a three-under 70 to join Adam Scott at three-under-par 143.