HILTON HEAD ISLAND, South Carolina, April 14, 2012 (AFP) - Pettersson also sank a seven-foot birdie putt on the 18th hole to nudge ahead of American Colt Knost for the lead after 54 holes at the $5.7 million event, standing on 12-under par 201 with Knost another shot back.
"It was tough," Pettersson said. "The wind died down and got back up. The greens were quick. I played a really good round. I'm happy. I was able to finish strong."
A host of other US challengers lurked with Zach Johnson third on 205, Boo Weekley fourth on 206 and Kevin Na, Robert Garrigus and Brendt Snedeker all on 207 and Tommy Gainey sharing eighth on 208 with England's Brian Davis.
England's Donald is on the verge of handing the world's top ranking back to Rory McIlroy on Monday without a major charge up the leaderboard in the final round.
Donald, who fired a par-71 on Saturday to stand on 215 in a share of 52nd place, must finish no worse than a share of eighth to stay ahead of the reigning US Open champion.
"Now that I've been No. 1 for a number of weeks now, it's not something I think about too much," Donald said.
McIlroy, who has the week off, will make his second reign in the rankings barring a reversal by Donald, taking the top spot for the first time last month after he won the Honda Classic, only to lose it to Donald two weeks later.
Pettersson, however, is more concerned about winning the $1.026 million top prize and his fifth career US PGA title, the first since he captured the 2010 Canadian Open.
"I have to play my game and stay calm," the Swede said. "I feel like if I play well I have a good chance."
Pettersson began his birdie binge at the par-5 second hole and also sank a long putt at the par-5 fifth during the run. He opened the back nine with a birdie but took bogeys at the 11th and par-5 16th, the latter after finding a bunker on his approach.
Knost sandwiched an eagle putt from off the green at the par-5 second between two bogeys, birdied the fifth and ninth, began the back nine with a bogey but answered with a birdie at the par-4 13th to stay on the Swede's heels.
"I got off to a little bit of a rough start. The putter saved me," Knost said. "It wasn't a great day but I'm looking forward to tomorrow. It wasn't my best but I feel like I can be ready for tomorrow."
Knost, who began his US PGA career in 2007 yet managed his first top-10 finish just seven weeks ago, sharing third at the Mayakoba Classic in Mexico.
Leading third-round scores on Saturday from the $5.7 million US PGA Heritage (USA unless noted, par-71):
201 - Carl Pettersson (SWE) 70-65-66
202 - Colt Knost 67-66-69
205 - Zach Johnson 71-68-66
206 - Boo Weekley 70-66-70
207 - Kevin Na 70-68-69, Brandt Snedeker 71-67-69, Robert Garrigus 71-66-70
208 - Tommy Gainey 70-70-68, Brian Davis (GBR) 72-68-68
209 - Billy Mayfair 72-70-67, Michael Bradley 74-64-71, Chad Campbell 67-70-72, Harris English 68-68-73
210 - Cameron Beckman 73-71-66, Jim Furyk 68-75-67, Kevin Stadler 72-71-67, Rory Sabbatini (RSA) 70-72-68, Matt Bettencourt 73-69-68, Jason Bohn 70-71-69, Matt Kuchar 72-69-69, Vaughn Taylor 67-73-70, Chez Reavie 69-71-70
211 - Stephen Ames (CAN) 71-74-66, Jason Dufner 78-66-67, Michael Thompson 71-70-70