Sweden's Fredrik Jacobson carded a 3-under 68 in round two and is alone in third place at 7-under-par 135. Billy Andrade (70) and Joey Sindelar (68) are tied for fourth place at minus-6.
Singh drained a 40-footer for eagle at the hole to reach 5 under par for the championship.
'I made an eagle on No. 9 that really relaxed me and just gave me a really good mood to go out there and play relaxed golf on the back nine,' said Singh, a three-time major winner.
Singh continued his strong play at the 10th. He rolled in a 5-footer for birdie at the par-4 hole, then parred his next three holes, but had several decent looks. Singh came up shy of the green at the 11th and his chip just came up short. One hole later, Singh's look birdie try just missed the hole on the right.
At the par-3 14th Singh got back on track with the flat stick. Singh hit his tee ball to 13 feet and drained the birdie putt to get to minus-7. At the 16th, Singh played conservatively short of the flagstick. His 40-foot birdie putt moved just right at the hole, but Singh was tied for the lead at 7 under par.
Singh nearly landed on the green with his second at the par-5 closing hole. His chip from the right fringe came up 7 feet short, but Singh converted the birdie try to move into sole possession of first.
'I managed to play a good round yesterday, and today I went out there and was struggling in the beginning, not struggling, but hit a few shots over the green,' admitted Singh. 'I just started making some putts and putting good shots together.'
Singh is mired in something of a slump. The former No. 1 player in the world has not won on tour since last July's Buick Open. He lost a playoff at the season-opening Mercedes Championships, posted seven top-10s, but has yet to really contend for a title since Hawaii.
'This year has been horrible, really,' acknowledged Singh. 'I don't know if it's my game or it's my head. I've been very impatient with my golf game. You cannot play the game of golf from the long grass, and that's what I've been doing. Today finally I started hitting fairways and seeing a lot of good positive things.'
Howell, the European Tour's Order of Merit leader, who won the BMW Championship two weeks ago, traded two birdies and two bogeys over his first 10 holes.
At the 11th, Howell ran home an 8-foot birdie putt to get to 6 under par. He dropped a shot at the 12th, but rebounded with a 10-footer for birdie at 13.
Howell hit an 8-iron 40 feet short and right of the target. He sank the long birdie putt to get to minus-7, then two-putted for birdie from 28 feet at the last to join Singh in the lead at 8 under par.
'I felt like I played reasonably nicely, but I probably had a lapse in concentration,' said Howell. 'I'm playing with Vijay tomorrow, which is great. It's going to be a big week for me.'
First-round leader Adam Scott struggled to a 1-over 72 on Friday. He is tied for sixth place with Brett Quigley (66), Luke Donald (65), Frank Lickliter (67), Ben Crane (66), Kevin Sutherland (67), Pebble Beach champion Arron Oberholser (69) and WGC-Accenture Match Play Championship winner Geoff Ogilvy (68). The group is knotted at 5-under-par 137.
Reigning Masters champion Phil Mickelson posted his second consecutive 1-under 70 and is part of a group tied for 29th place at minus-2.
The 36-hole cut fell at 1-over-par 143 with 84 players advancing to the weekend.
One player who won't be teeing it up on the weekend is Sergio Garcia. The two- time winner of this event hurt his back and pulled out before he made the turn on Friday.
'I hurt my back Wednesday practicing and playing in the Pro-Am, and struggled with it the last two days,' said Garcia, who expects to play in next week's U.S. Open. 'After nine holes today, I wasn't able to continue.'