He was one shot ahead of Mickelson, who made another great escape at the end of the second round, this time holing a 20-foot par putt to finish off a 66 that put him in the final group with Singh.
Both have three majors - two Masters and a PGA for Mickelson, two PGAs and a Masters for Singh
- along with some history. They got into a heated argument during a rain delay at Augusta National over the length of Mickelson's metal spikes. A year later when they played two rounds together in Phoenix, Singh asked that Mickelson's driver be tested to make sure it was legal.
Both have more pressing concerns this week, mainly getting their games back in order with the PGA Championship looming.
"I'm going to go out there and play my heart out and try to shoot as low as I can, and not really be concerned about what Phil does," said Singh, who was at 7-under 133. "He's going to be focused on his game. I just hope we both have a good day."
Sean O'Hair, seeing immediate results from switching to a new swing coach, had a 67 and joined the group at 5-under 135 that included Lee Westwood (65), former Masters champion Zach Johnson (68) and Peter Lonard (66).
Sixteen players were within four shots of the lead.
"It's anybody's ball game," said Hunter Mahan, who had a 66 and was at 3-under 137.
It helps that Tiger Woods isn't at Firestone, particularly since he is a six-time champion on this course and had never finished out of the top five in his 10 appearances.
"To me, him not being here is the difference between 39th and 38th," Lonard said with a laugh.
Singh was the last player other than Woods to be No. 1 in the world, a 32-week reign in 2004-05. But he now is 45, coping with nagging injuries and a victory drought on the PGA Tour that has lasted 18 months and caused him to fall to No. 15 in the world ranking.
The culprit? He blames his putter.
Singh got so fed up with his conventional putter when he missed the cut at the British Open that he went back to the belly putter during a week of practice and swears he will stick with it.
"I'm not a great putter, but I'm not a bad putter," Singh said. "The British Open was the turning point, where I played really well and putted really badly, and decided that's it. I'm not a good putter with a short putter. I've decided that I'm going to putt with the belly. If you see me with a short one, that means that something is wrong with me."
The only thing wrong on a balmy Friday at Firestone was his driver, although it didn't hurt him too badly. One day after missing only one fairway, Singh was in the short grass six times in the second round, and needed to work his shots around the tree-lined fairways on the sixth and eighth holes at the end of his round to avoid dropping shots.