Garcia ended the round at 13-under-par 129, one stroke better than a pair of competitors.
Sweden's Henrik Stenson fired an 8-under 63 to catch up with Roberto Castro (65) to share second place at minus-12 at TPC Boston.
U.S. Open winner Justin Rose (63), Jordan Spieth (66) and Harris English (67) finished 54 holes one shot farther back.
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