The Englishman, three times a runner-up at this event, mixed four birdies with two bogeys to post a seven-under total of 206, ending the round a stroke in front of Americans Charley Hoffman (71) and long-hitting Jason Kokrak (68).
Australian world number one Jason Day, joint leader overnight with Hoffman and fellow American Kevin Chappell, tumbled down the leaderboard with an eight-over 79 as he appeared to succumb to fatigue in his eighth event of the year.
Donald, who has not tasted victory since the 2013 Dunlop Phoenix in Japan, was delighted to keep a relatively clean scorecard at gusty Harbour Town as the greens firmed up in the afternoon sunshine.
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"It's probably one of the toughest days I have had out here, condition-wise," Donald told Golf Channel after saving par on the tough finishing hole with an exquisite chip from 25 yards to inside three feet.
"Coming down the last few holes, the greens were getting firmer and faster and there were a couple of really tough pin positions. It was tough to be aggressive and make a lot of birdies.
"A poor iron (approach) into 18 but, other than that, I felt like I hit a lot of good shots and I was very happy with that score," said world number 95 Donald, who has not triumphed on the PGA Tour since the 2012 Transitions Championship.
Seven players held at least a share of the lead during a wildly fluctuating third round -- Donald, Hoffman, Kokrak, Day, Chappell, Bryson DeChambeau and Russell Knox.
Hoffman, seeking a fourth PGA Tour victory, briefly stormed two ahead with a birdie-birdie start before he ended a topsy-turvy round with a bogey-bogey finish.
"I have been getting in my own way so hopefully I can just reach down deep. I know I've done it (win) before, so just try and do it again," Hoffman said of his thoughts heading into Sunday's final round.
Day, bidding for a seventh tournament win in just 14 starts, never got his round back on track after running up a double-bogey at the par-four third where his tee shot found water.
"I knew something was up when my ball hit a tree and bounced 80 yards right into the hazard," said Day. "From there, I just got a couple of bad breaks.
"It's really tough to fight back after that ... just got to try and improve tomorrow and see how it goes."
(Reporting by Mark Lamport-Stokes in Los Angeles; Editing by Andrew Both)
