SAN FRANCISCO -- Anthony DeSclafani hadn’t allowed more than seven hits in a single start all year, but he matched that total in the first inning alone on Monday night.

DeSclafani was tagged for a season-high five runs in a rocky first, allowing the Nationals to cruise to a 5-1 series-opening win over the Giants at Oracle Park.

San Francisco couldn’t overcome the early deficit, delivering a flat offensive performance against Nationals right-hander Jake Irvin, who gave up only four hits over 6 1/3 scoreless innings in his second career Major League start.

The Giants (15-19) grounded into four double plays and didn’t get on the board until Joc Pederson led off the bottom of the ninth with a solo shot to the opposite field off reliever Andrés Machado. They’re now 2-12 when scoring fewer than three runs in a game this year.

“I don’t think we had our best day at the plate,” manager Gabe Kapler said. “Obviously, you’re putting the ball on the ground a lot, and they’re able to convert those into double plays. We’re going to have to get the ball in the air more frequently, continue to work counts and swing at pitches we can drive. We’ll be better than we were tonight.”

DeSclafani entered Monday with a 2.13 ERA and was coming off his best start of the year after firing eight shutout innings against the Astros last week, but he was surprisingly hittable early on against the Nationals. 

Lane Thomas and Luis García led off with back-to-back singles to put runners on first and second with no outs. Keibert Ruiz followed with another single to right field, though DeSclafani caught a break when Thomas hesitated while advancing from second to third, keeping García at second and allowing the Giants to catch Ruiz in a rundown for the first out of the inning.

Joey Meneses added a fourth consecutive single to put the Nationals on the board, and DeSclafani couldn’t limit the damage after that, yielding three straight two-out hits to Dominic Smith, Alex Call and CJ Abrams to sink the Giants into a 5-0 hole. DeSclafani ended up facing nine hitters, though he threw only 21 pitches in the fateful inning. 

“He was just catching a little bit too much of the plate early in the game, and it’s a team across the way that’s pretty aggressive,” Kapler said. “Not just aggressive on fastballs, aggressive on all pitches. There are going to be times when he takes that same exact approach, and those balls are going to be at people. They did a nice job, giving them credit, swinging the bats early on. 

“Obviously, we’d like to start the game over and do the first inning again, but other than that, I thought he pitched fine.”

After the rough start, DeSclafani convened with pitching coach Andrew Bailey in the dugout and learned that he wasn’t getting his optimal shapes on his slider and his two-seamer. He made a mechanical adjustment between innings, tweaking the way he took the ball out of his glove, and soon regained the form that has turned him into one of the Giants’ most effective starters this year. 

DeSclafani, whose ERA rose to 2.80, allowed only three hits over the next six innings, enabling the Giants to use only one reliever -- rookie Tristan Beck -- to cover the final two frames.

“They’re an aggressive team,” DeSclafani said. “But you can always use that to your advantage. You kind of saw that the rest of the game. I kind of got my shapes back, and you use the aggressiveness of the hitters to skate through some quick innings. 

“It’s kind of tough to give up a five-spot there. The job is always to go deep into the game, so I’m not going to let a crooked number or a big inning kind of deter that.”

DeSclafani was tested again when the Nationals put a pair of runners on with two outs in the seventh, but he managed to escape the jam with the help of Mitch Haniger, who made a diving catch in left field to rob Jeimer Candelario and end the inning. 

“That was really cool,” DeSclafani said. “Crazy. Really good play. I loved it.”