With Tiger Woods and Rory McIlroy hogging the headlines through the opening two rounds, it was Mickelson's turn to take centre stage, marching to a career high with 10 birdies in an eight-under 64 to join Singh on 16-under.
"I got it going with the putter," Mickelson told reporters. "It was very similar to yesterday's round, I played really well and had a lot of good opportunities.
"But yesterday when the ball was getting to the lip and turning away, today they seemed to fall in."
Singh, chasing his first PGA Tour win in four years along with a ticket to the season-ending Tour Championship, had grimly held on to top spot alone for much of the day but wobbled badly coming home.
The big Fijian, who needs to finish fourth or better on Sunday to clinch a FedExCup berth, mixed two birdies with three bogeys over his final five holes, including one at the last, to settle for a three-under 69.
"I've got a good chance tomorrow to do it," said Singh, who has 34 career PGA Tour wins but has not been in the winner's circle since 2008. "Today's round could have really gotten away from me but I really dug deep and really focused hard."
With only 30 of the 70 golfers teeing off at Crooked Stick Golf Club this week able to qualify for the Sept. 20-23 Tour Championship and a shot at the $10 million bonus, there was no shortage of players making big moves.
Briton Lee Westwood, who has never qualified for the Tour Championship, helped his chances with a four-under 68 to enter Sunday's final round alongside world number one McIlroy (69), one stroke adrift of the leaders.
"I'm only one back," said McIlroy, who has won two of his last three starts. "It wasn't the greatest ball striking round that I have ever had but somehow managed to scrape it around in 69 and still in with a chance."
Australian Adam Scott (66) and Americans Robert Garrigus (66) and Dustin Johnson (67) sit two off the pace at 14-under with Woods lurking one shot further back after grinding his way to a one-under 71.
Golf fans could be in for a special treat on Sunday with a major championship quality leaderboard that features six major winners and four players who have held the number one ranking among the top 10 ready to slug it out.
"I've played with them forever," said Singh, who will celebrate his 50th birthday next February. "They'll be just another player out there for me tomorrow.
"I think I'm playing good enough to win the golf tournament and that's how I am going to think tomorrow."
The start of play was delayed several hours as course workers laboured to get the layout playable after an overnight storm dumped almost three inches of rain on Crooked Stick.
With the ultra-soft conditions and golfers able to lift, clean and place their balls, players attacked the pins with gusto and were on the move right from their opening shots.
Mickelson stumbled out of the blocks with a bogey at the third but quickly shifted into top gear, reeling off seven birdies over his next 10 holes, including four straight from the sixth to jump to the top of the leaderboard.
After a bogey at the 13th, Mickelson wrapped up his round in style with three more birdies over his final four holes, including 17 and 18.
As Mickelson surged, Woods faltered.
Four bogeys over his opening eight holes sent him diving down the leaderboard before he halted the slide by chipping in front of the fringe for a birdie on the ninth. He picked up shots at 10 and 11 to get back to level par on the day.
The 14-time major winner added another birdie at the 13th but signed off with five straight pars.
"I grinded hard," said Woods, who along with McIlroy goes into Sunday looking for a fourth PGA Tour win of the season. "I didn't hit any good shots.
"I'm within reach.
"It's probably going to take 63 or 64 tomorrow but it can be done out here."
- Tiger Woods
- Rory McIlroy
- PGA Tour