Singh twice escaped from the trees on his closing holes at Firestone and renewed his affair with a belly putter on his way to 4-under-par 66 at the Bridgestone Invitational, putting his name atop the leaderboard for the first time in more than four months.
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 round of 66 that put him in the final group with Singh.
Both have won three majors -- two Masters Tournament titles and one PGA Championship 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 Golf Club 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, 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 67 and joined the group at 5-under that included Lee Westwood (65), former Masters champion Zach Johnson (68) and Peter Lonard (66).
Augusta native Charles Howell was five back, at 2-under, after he followed his opening 68 with 70. Evans resident Vaughn Taylor is at 1-under after 67.
Augusta State alum Oliver Wilson shot 69 and is seven shots back.
Sixteen players were within four shots of Singh's lead.
"It's anybody's ball game," said Hunter Mahan, who had 66 and was at 3-under.
Singh was the last player other than Woods to be No. 1 in the world, a 32-week reign in 2004-05. But Singh, now 45, is 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.
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."
Mickelson hit his share of errant shots, mostly on his approach to the greens. But the par save on his final hole gave him some momentum going into the weekend.
"To see balls rolling in now, I'm starting to gain a little bit more confidence, a little bit more momentum," Mickelson said.