Lester (11-7) scattered six hits and two walks, and struck out three on his way to snapping a personal three-start winless streak.
Shane Victorino and Daniel Nava each provided three hits with an RBI, while Jarrod Saltalamacchia and Will Middlebrooks also drove in one run each for the Red Sox, who retained a one-game lead over Tampa Bay atop the American League East.
Tim Lincecum (6-13) was roughed up for five runs on nine hits in a six-inning effort as the Giants fell in the opener of a three-game set.
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