Harrington picked up a pair of bonus birdies over the last five holes for 6-under-par 65, his low score of the year, and built a one-shot lead going into the weekend on the tough Copperhead course at Innisbrook.
The Irishman has not won a sanctioned tournament since his PGA Championship victory at Oakland Hills at the end of 2008.
"I'm capable of winning in the form I'm in," said Harrington, who was at 8-under 134. "Am I in my best form? No. Am I getting there? Yes. I'm kind of in the form that I was in at the end of last year. I'm right in the midst of it."
And he had lots of company.
Jim Furyk, whose last victory against a full field came in the 2007 Canadian Open, was atop the leaderboard with Harrington until missing a 6-foot par putt on the final hole. It wasn't enough to take away from an otherwise solid round of 68 to put him one shot behind.
Steve Stricker played the final eight holes in even par and still shot 66, putting him in the group two shots behind.
Stricker, the highest-ranked player at Innisbrook at No. 2 in the world, was poised to catch Harrington until he hit driver on the 16th hole to make sure he cleared the water, went too far and had his ball land next to a tree, leading to bogey.
Harrington spent so much of last year trying to find a swing key that he was rarely in contention until August. He started this year with a stronger focus on scoring, and this is his best opportunity yet.
He doesn't know the Copperhead course as well as others, but so far that hasn't hurt him.
Harrington allowed himself a distraction by taking a whirlwind trip to Washington on Wednesday to celebrate St. Patrick's Day with President Obama and Ireland's prime minister.
Harrington wouldn't trade the experience, even though he didn't greet the president.
"If I thought going to the White House was going to detrimentally affect my performance, one or the other would have to be given up, either the tournament or the White House," he said.