BOSTON - Gio Gonzalez pitched 6 1/3 strong innings to outduel Daisuke Matsuzaka and Adam LaRoche homered as visiting Washington clinched a series win at Fenway Park with a 4-2 win over the Boston Red Sox.

Ian Desmond singled in a pair and Michael Morse added an RBI double for the Nationals, who have taken the first two games at Boston and four of their last five overall.

The Red Sox have dropped five of six to fall back below .500 at 29-30.

Gonzalez, who entered with a 5.79 ERA in five career starts against Boston, dominated through the first six innings, allowing two hits and striking out five. The left-hander was lifted after allowing a walk and a single in the seventh and was charged with two runs when reliever Mike Gonzalez yielded a two-run single to pinch hitter Jarrod Saltalamacchia.

Matsuzaka, who was making his season debut after recovering from Tommy John surgery, struck out eight in five innings but was charged with four runs on five hits while walking one.

LaRoche opened the scoring in the second, driving a solo homer into the Boston bullpen in right.

The Nationals rallied for three runs in the fourth, with Morse sending a ground-rule double down the line in right and Desmond following with a two-run single to right.

Tyler Clippard worked the ninth for his seventh save.

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GAME NOTEBOOK: To clear a roster spot for Matsuzaka, Boston designated CF Marlon Byrd for assignment. ... After drawing cheers from the Fenway crowd with a 3-for-5 performance on Friday, Nationals rookie Bryce Harper went 0-for-3 with a pair of strikeouts and a walk on Saturday. ... Red Sox LF Carl Crawford, who has been out with wrist and elbow injuries, is working on a new throwing motion to take strain off the elbow. ... Desmond has four RBIs in the first two games of the series.