Phil Mickelson equalled the course record with a eight-under-par 64 to surge into a share of the lead alongside Vijay Singh in the PGA Tour BMW Championship in Carmel, Indiana.
The four-time major winner had 10 birdies, offset only by bogeys at the third and 14th holes, and will head into Sunday's final round tied with Fijian Singh on 16 under par.
Midway leader Singh carded a third-round 69, which would have looked much healthier had he not bogeyed three of his last five holes.
The pair were a shot clear of England's Lee Westwood and Northern Ireland's world number one Rory McIlroy after 54 holes.
Westwood posted a four-under 68 on Saturday, spurred by three birdies in his closing four holes, with PGA champion McIlroy signing for a 69.
Tiger Woods, who was level with McIlroy a stroke behind Singh heading into the third round, slipped off the pace with a 71.
The 14-time major winner, who had four bogeys on his front nine, will start Sunday on 13 under and in eighth place - a shot behind fellow Americans Robert Garrigus (66), Dustin Johnson (67) and Australia's Adam Scott (66).
Speaking after his sparkling round, Mickelson expressed his delight at the form he has shown over the past two weeks.
"Certainly my game went south for a while, and I was trying to piece it back together," said the American left-hander, who finished joint fourth at the Deutsche Bank Championship in Norton on Monday.
"I started with some iron play and it progressed to getting my driver back to where I was hitting the ball straight and getting my speed back, and now the putter is back.
"It's taken me a little while to piece it back together, but I could tell last week that my game was back and I was ready to play at the highest level again.
"I knew heading into this week I was going to have a good week. I came out very anxious on Thursday and forced it a little too much the front nine and didn't play a good nine holes.
"But after that I've been able to settle down and play some good golf. I'm looking forward to tomorrow's shootout. It should be fun."
McIlroy admitted he had not been at his best but was content with his position heading into Sunday.
"It wasn't the greatest ball-striking round out there that I've ever had, but I somehow managed to scrape it around in 69 and still in with a chance," the 23-year-old said.
"As badly as I did hit it out there, I'm still happy with my position going into tomorrow."