Swede Henrik Stenson's 66 took him within two shots of Garcia while Canada's Graham DeLaet produced a sparkling nine-under 62 to sit one further adrift along with American Steve Stricker (63).
A course which had already offered low scores was even more benign after morning rain left the greens even softer. So often the talk in golf is of creating tougher and tougher challenges for players but the fans who ventured out after the storms that delayed play enjoyed seeing the world's best in a 'Birdie Fest'.
On a day when only 11 players shot over par, Garcia appeared particularly, if unusually, relaxed in a tournament he had not initially planned to play in.
Garcia's much-publicised "fried chicken" jibe at Tiger Woods in May and the trouble it caused him seemed a distant memory as he strode confidently around the wet TPC Boston, drawing warm cheers from the New England crowd with seven birdies.
"We can't control the weather," said Garcia, whose bogey on the 14th was his sole blemish. "And you've just got to go out there and try to play the best you can. And I was very happy to see that my best was six-under today," he said.
"Hopefully I'll be able to believe in (myself) in the way I've been believing this whole weekend. If I'm able to do that I should have a chance, a good chance of winning.
"If not, then I'll fight as hard as I can to get it," he added. DeLaet's score was the best of the week and continued his purple patch following his impressive tie for second at the Barclays last week.
"It was an incredible day," said the Canadian. "It's one of those days that it doesn't come along as often as you'd like. But it was one of those days where I was seeing the lines great and made pretty much every putt that I would expect to make, along with a couple of 20-footers."
American Jason Dufner, who fired six birdies in his opening 10 holes, was unable to build on his excellent start, but his 66 left him four strokes behind Garcia on 14-under alongside compatriot Roberto Castro (68).
After enjoying much of the limelight as playing partners for the first two rounds, world number one Tiger Woods and British Open champion Phil Mickelson had disappointing days.
Mickelson had three bogeys on the back nine and his second straight even-par round left him 11 behind Garcia. Woods found the fairway just seven times in his one-over 72 to be six-under, tied in 47th place.
"I just didn't have it today," Woods lamented. "I just didn't hit it well. I didn't make anything. Just one of those days - I had a bad day at the wrong time."
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