Garcia had a two-stroke lead over American Roberto Castro who still had two holes to complete but suffered a setback with a double bogey on the 14th - the same hole that Garcia also doubled.
England's Brian Davis, the joint overnight leader with world number three Phil Mickelson, fell to six back of Garcia after he was one over through 13 holes.
Mickelson is due to start his second round, along with world number one Tiger Woods and Masters champion, Australian Adam Scott at 1:10 p.m. ET (1710 GMT).
Garcia made a blistering start to his round, with some superb ball-striking, making five birdies and no bogeys as he reached the turn in 31.
Soft greens made for low scoring again at TPC Boston in the second tournament of the four-event FedExCup playoffs.
(Reporting By Simon Evans,; Editing by Gene Cherry)
