Brandon Belt rounds the bases after his home run off Stephen Strasburg in the sixth inning of the Nationals’ 7-3 loss to the Giants. (John McDonnell/The Washington Post)

Stephen Strasburg appeared to be headed toward his best outing of the season. The Washington Nationals starter mowed through the first six San Francisco Giants hitters he faced Tuesday night, his fastball steaming at 95 mph, his change-up keeping them off balance. He gave himself the lead with an RBI double in the second and protected it when the first hiccups came, a double allowed in the third, a sharp single in the fourth. Strasburg pounded the zone and mixed in his off-speed stuff and steered around danger. Then came the fifth.

Evan Longoria led off the inning, and he crushed Strasburg’s two-seam fastball beyond the center field fence. The once-promising start began to slip away before derailing in what became a 7-3 Nationals loss at Nationals Park.

Two batters after Longoria’s home run, Strasburg allowed a single to Gerardo Parra. Two batters after that, Steven Duggar belted another two-seamer that caught too much of the plate for a two-run homer and a 3-1 Giants lead. Many pitchers struggle with runners on base, but Strasburg presents a special case. According to tOPS+, an advanced metric that compares a pitcher’s performance with men on base with his performance without men on base, Strasburg is the sixth-worst pitcher in the major leagues with men on among 53 active pitchers with at least 1,000 career innings pitched.

In short, Strasburg struggles to stop bad innings from snowballing.

“I wouldn’t really say, like, ‘piled up,’ ” Strasburg said of the runs he allowed. “A guy put a good swing on the ball. It was just one of those nights.”

Nationals Manager Dave Martinez saw Strasburg wanted to go in with those two-seamers to Longoria and Duggar and appreciated the approach, but he believed Strasburg needed to “stay with the plan” to prevent big innings. This is not the first time they have talked about sticking to the script, Martinez emphasized after the game.

“He had a good plan going into the game,” Martinez said. “Those two pitches, he got away from it a little bit. He’s just got to continue to stay with the plan.”

Strasburg surrendered another home run in the sixth, this one to first baseman Brandon Belt, which meant Strasburg had given up three home runs in a game for the first time since June 2017. Suddenly, a Giants offense that had been no-hit through eight innings Sunday in its previous game, that entered with the lowest team on-base percentage in the majors (.259), and that had hit 10 home runs all season, looked like a juggernaut.

Strasburg was lifted after the sixth, having allowed four runs on six hits with eight strikeouts and no walks. Washington’s bullpen did nothing to quiet concerns about its effectiveness as San Francisco (8-10) added two runs in the seventh and another in the eighth.

“It’s important to just kind of stay focused, focus on the things that you’re working on, focus on the process,” Strasburg said. “It’s” — he checked his watch — “April 16. There’s a lot of season left, and a lot of things can change very fast.”

The loss was a discouraging start to a grueling stretch for the Nationals (7-8), who embarked Tuesday on a run of 26 games in 27 days across six cities and four time zones.

Strasburg was outpitched by Giants starter Dereck Rodriguez, whose father, Hall of Famer Ivan Rodriguez, caught Strasburg’s major league debut nine years ago. The Nationals looked as though they would chase Rodriguez early, drawing two walks in the first inning and getting back-to-back doubles by Wilmer Difo and Strasburg in the second to take a 1-0 lead, but, in a situation that is becoming familiar, they couldn’t deliver the knockout blow. Rodriguez settled in and did not allow another run through his five innings of work.

“He doesn’t light up the radar gun, but he knows how to pitch,” Nationals catcher Kurt Suzuki said. “He knows how to move the ball around, and pitchers that do . . . find times to make a pitch. That’s what he did tonight.”

In the bottom of the fifth, with the Nationals now trailing after Strasburg’s rough inning, hot-hitting third baseman Anthony Rendon came to the plate with a runner on first and two out. Martinez had voiced his frustration with the umpiring all night — mostly about balls and strikes — and when Rendon struck out looking on a change-up that looked too high, the manager said he yelled, without cursing, “You got to be better than that,” at home plate umpire Tony Randazzo.

On the spot, Randazzo gave Martinez his second career ejection.

“What really irritated me was him putting his hand up in my face,” Martinez said. “I can tolerate a lot of things. Don’t do that. I have a lot of respect for umpires — everybody knows that. I typically don’t complain too much about them. But, you know, him walking towards our dugout when I’m in the dugout. . . . I hope the league looks at that because, like I said, I didn’t say much to really get tossed.”

Martinez charged from the dugout steps and onto the field. He barked at Randazzo, and then he barked some more.

When Martinez finally strode back toward his dugout, the two-run deficit somehow seemed larger. For the Nationals, this was supposed to be a series to establish themselves and spur an early-season run by dispatching a middling ballclub that had just flown across the country hoping for a spark. The series could still turn out that way, but on Tuesday, the Nationals looked like the team in need of a jump-start.