Williamson, promoted from the minors last week when Hunter Pence was placed on the disabled list, drove in three runs, two with his second homer in his first 13 at-bats in the majors this season.

The win in the opener of a 10-game homestand was the Giants' third in their past four games.

Stratton and three relievers combined on a five-hitter as the Giants handed Washington its fourth loss in five games.

Stratton (2-1) remained unbeaten since losing his season debut on April 1, allowing two runs and four hits in 6 2/3 innings Monday. He walked three and struck out five. The 27-year-old has allowed two or fewer earned runs in each of his past four starts.

Giants pitching, meanwhile, held the opponent to four or fewer runs for the seventh consecutive game.

Sam Dyson, Tony Watson and Hunter Strickland threw 2 1/3 innings of one-hit, shutout relief after taking over for Stratton. Strickland, who pitched around a leadoff walk in the ninth, was credited with his fourth save.

Other than their two-run sixth, the Giants also scored single runs off Nationals starter Gio Gonzalez in the fourth inning on a Williamson infield out and the fifth on an RBI single by Buster Posey.

Gonzalez walked Brandon Belt to open the sixth, then handed the ball over to Shawn Kelley, whose first pitch was taken out to right-center field by Williamson for a 4-1 lead.

Gonzalez (2-2) was charged with three runs in five-plus innings. He allowed four hits and three walks, and struck out four.

Howie Kendrick and Andrew Stevenson drove in Washington's runs with sacrifice flies.

The Nationals, once again without injured Adam Eaton, Daniel Murphy and Anthony Rendon, got two hits apiece from Trea Turner and Ryan Zimmerman.

Turner scored Washington's first run in the third inning after hitting a triple, the club's only extra-base hit of the night.

Posey had two hits, including a double, for the Giants, who managed six hits.

--Field Level Media