Boston’s Kelly Shoppach is dismayed to realize he has just struck out looking against Gio Gonzalez in the fifth inning. Gonzalez fanned five in 6 1/3 innings. (Jessica Rinaldi/Reuters)

The Boston Red Sox have rapidly discovered the most essential quality these Washington Nationals possess. They do not let up. They will throw Stephen Strasburg and a 19-year-old cyclone at you one night, and the next day the league’s least hittable pitcher and most athletic infield will be waiting. They never let you catch your breath. They give no quarter.

It is the primary reason why the Nationals have the second-best record in the major leagues, and why they held on to beat the Red Sox, 4-2, at Fenway Park on Saturday afternoon. Gio Gonzalez backed up Strasburg’s dominance with a gem of his own, allowing two runs in 61 / 3 innings on three hits and two walks. The dual brilliance that clinched a series victory has defined their season. The last two days, Strasburg and Gonzalez struck out 18 while allowing seven hits. The Nationals are 20-4 when Strasburg or Gonzalez start.

“Oh, I’ve heard it all year from other teams,” first baseman Adam LaRoche said. “Guys get down to first and say there’s absolutely no break in this rotation. And it can be a miserable feeling. You run into an offense that’s struggling a little bit, and our staff definitely doesn’t help.”

Saturday, the Nationals’ final 15 batters failed to reach base, but they produced enough offense off Red Sox starter Daisuke Matsuzaka, who made his season debut after undergoing Tommy John surgery. LaRoche clobbered his 10th homer of the season. Michael Morse drilled an RBI, ground-rule double. Ian Desmond smacked a game-breaking, two-run single.

The Nationals needed the cushion as Gonzalez faltered late and the Red Sox rallied. But for the most part Gonzalez dominated, rebounding from a rare clunker against the Atlanta Braves in his last start.

Gonzalez entered with a league-leading 5.4 hits allowed per nine innings, having yielded two or fewer in five of 11 starts. Then he retired 12 of the first 13 batters he faced, only four of those balls leaving the infield.

“His curveball was late and he was throwing it for a strike,” Red Sox second baseman Dustin Pedroia said. “His fastball’s really good, too. He’s 95 with movement. He’s got great stuff. Both those last two guys we’ve faced are No. 1s. The last two guys were pretty darned good.”

The Nationals’ infield defense adds another layer to the challenge. In their second year together, Desmond and second baseman Danny Espinosa have become one of the best middle infields in the majors.

“Those two are incredible,” LaRoche said. “There’s been a lot of balls where, especially early on, I kind of gave up on thinking it was a base hit. I’d find myself having to bust it over there [to first]. Now I know if it’s on the ground, I better get there.”

Saturday, Desmond made two sparkling plays. He charged one slow roller and made a smooth, running throw. In a shift against David Ortiz, Desmond sprinted to the right side of the infield and made a throw across his body for the out. Espinosa added a leaping, full-extension catch to rob a single with a runner on second. Teammates have traded awe at their highlights for shock when they don’t make the play.

“You can get caught being tentative out there, like, ‘Ah, they’re going to get there,’ ” center fielder Rick Ankiel said. “Then all of a sudden, it gets by them. You’re like, ‘Oh, [shoot].’ ”

Gonzalez carried his shutout into the seventh. With one out, he walked Will Middlebrooks. Mike Aviles’s single, combined with a bobble by Bryce Harper in right field, put runners on first and third with one out.

Manager Davey Johnson typically allows Gonzalez to fight for his own earned runs. In Cincinnati this year, he threw his 115th pitch with the bases loaded. But Johnson saw troubling signs. In the sixth, Gonzalez had failed to cover first base on a grounder to LaRoche, his second such foible in as many starts. Johnson saw Gonzalez’s delivery speeding up and his control waning.

Johnson removed Gonzalez after 98 pitches. Gonzaelz bit into his glove as he left the mound, and he screamed into the mitt while he walked off the field.

“I knew I left myself wide open to be second-guessed,” Johnson said. “I’ve seen him get in those situations where it’s like he’s trying to get to the finish line. I’ve stayed with him numerous times, but I didn’t have that good feeling in this ballpark.”

Said Gonzalez: “No frustration. My arm slot was just going down a little bit. I couldn’t find the zone after that.”

In came Craig Stammen, who walked pinch-hitter Ryan Sweeney to load the bases. Johnson removed Stammen immediately for left-hander Michael Gonzalez. Johnson wanted to turn around pinch-hitter Jarrod Saltalamacchia. He hits with more power from the left side, and a home run would have erased the Nationals’ lead.

The gambit kept Saltalamacchia in the park, but no more. He ripped the first pitch Gonzalez threw, a fastball down the middle, into the Green Monster. Two runners scored, slicing the Nationals’ lead in half.

Johnson stuck with Gonzalez, who struck out Daniel Nava with a clutch inside fastball and retired Pedroia to hold the Red Sox at two runs.

In the eighth, Johnson revealed why he had chosen Gonzalez, and not Sean Burnett, for the key assignment. Burnett, the Nationals’ stingiest reliever this year, was needed to face dangerous lefties Adrian Gonzalez and David Ortiz in the eighth. He induced weak grounders from both in a 1-2-3 inning, which lowered his ERA to 0.95.

Burnett “probably gets overlooked a little bit,” said closer Tyler Clippard, who converted his seventh save in seven tries since taking over the role. “He doesn’t flash up the gaudy miles per hour on the gun. But at the end of the day, you talk to these hitters, there’s no way they’re comfortable in the box.”

The Nationals’ had made another statement at the fabled park. They left their cramped clubhouse quickly, planning where to watch Game 7 of the NBA’s Eastern Conference finals. They will attempt a sweep Sunday. Jordan Zimmermann, owner of a 2.82 ERA, will start.

“It’s no picnic tomorrow,” Johnson said. For the Nationals’ opponent, it rarely is.