WASHINGTON -- Arriving in Washington last in the National League in batting average and home runs, the Giants looked headed for more of the same after four dominant innings from Nationals starter Stephen Strasburg.

Then all of the sudden, they weren’t.

Evan Longoria and Steven Duggar homered in the fifth, Brandon Belt added his fourth of the season in Strasburg’s sixth and final inning, and the Giants rode a rare power display to a 7-3 victory in Tuesday’s series opener at Nationals Park.

“Longo got that one, and I guess there was just more momentum after that,” said San Francisco starter Dereck Rodríguez. “We felt pretty good. Everybody had the right attitude coming into the game, the beginning of the game. And I guess that was just the tilting point I guess to keep going.”

Flying east after being one-hit in their series finale against the Rockies, the Giants raised their team average modestly above the Mendoza Line to .204.

“Hitting is contagious,” said Duggar. “It feeds off each other.”

San Francisco homered three times in a game for the first time all season against a power arm that had only allowed three homers twice in 209 previous career starts.

All three came on fastballs. Longoria and Dugger went opposite field. Belt clobbered his into the second deck in right. It was enough to hand Strasburg his first loss of the season, albeit in a slow April for him, pushing his ERA to 5.56.

“We had some very good at-bats against a very good pitcher,” said Giants manager Bruce Bochy, whose team has still only homered 13 times in 18 games. “It was good to see us break out after we just got one-hit. Good way to start this road trip.”

Rodriguez grinded through five innings, allowing only one run on Strasburg’s RBI double.

Gerardo Parra, Joe Panik and Kevin Pillar each drove in a run off Washington’s struggling bullpen, insurance that came in handy after Sam Dyson allowed two runs in the bottom of the seventh. Dyson’s performance snapped a stretch of 21 scoreless innings worked by San Francisco relievers.

Gaining control

Rodriguez admitted he struggled with command early, one reason he required 98 pitches to pitch as long as he did. But it improved as the night wore on, he said, and he finished his evening striking out Brian Dozier and Anthony Rendon looking at offspeed offerings.

Dozier took a cutter, Rendon a changeup. And Nats manager Dave Martinez took an early leave, ejected by plate umpire Tony Randazzo after arguing both calls had been generous.

“They just caught the corner, I feel,” Rodriguez said. “So, two strikes, expanded a little bit, and it went my way.”

Not Buster-ing out

Giants catcher Buster Posey was the only starter other than Rodriguez not to reach base, going 0-for-5. His average sank to .192, marking the latest in a season the six-time All-Star has hit below .200, save for when he debuted as a September callup in 2009.

Posey did catch two runners stealing, however, raising his caught-stealing average to 60 percent (6 of 10) for the year.