Stephen Strasburg allowed four home runs and six extra-base hits, both matching career highs. (Marlena Sloss/The Washington Post)

The Washington Nationals looked ready for a slugfest, using the first inning as their launchpad, matching the Arizona Diamondbacks’ early home run swings with a pair of their own.

But Stephen Strasburg was equally ready to take them out of it, one misplaced pitch at a time, in a 10-3 loss Saturday at Nationals Park. The starter had his worst outing of the season, and in recent memory, by giving up six runs on nine hits in five trying innings. He allowed four home runs and six extra-base hits, both matching career highs, and threw straight into contact while never finding his command.

“He couldn’t locate his fastball,” said Manager Dave Martinez, nodding to how consistent Strasburg had been. “Just missing his spots.”

The Nationals’ uneven play has returned since a 9-2 stretch pumped life back into their season. They have since lost five of nine, dropping them to 32-38, and have fallen nine games behind the Atlanta Braves in the National League East. (The Braves hosted second-place Philadelphia on Saturday night.) And this defeat, making it so the Nationals at best can only split this series with the Diamondbacks on Sunday, falls directly on Strasburg. He last yielded four home runs Aug. 8, 2014. He last yielded eight or more hits Aug. 17, 2016. He yielded two homers to Ketel Marte, the Diamondbacks’ leadoff hitter, and it was Marte’s bat that set the tone for Strasburg’s rough day.

Each game begins the same way in the Nationals Park press box: The first pitch is delivered, often a get-me-over fastball, and then the official scorer reads the time and weather over a loudspeaker. But there was a slight tweak Saturday afternoon, because Marte stalked Strasburg’s first-pitch fastball and drove it over the right field wall. And so the essential details — 4:06 p.m., 82 degrees when this one started — had to wait until Marte sped around the bases as the home crowd was still settling in.

“Um, no, not really,” Strasburg said when asked whether he was surprised by the Diamondbacks’ swing-happy approach. “I think when you make mistakes here in the big leagues, you give them a much better chance to barrel it. And they barreled it.”

Arizona’s second homer, a liner to left by Adam Jones, came three hitters later, but the Nationals dealt with that deficit as soon as they could. Trea Turner started their day at the plate with a triple, Adam Eaton brought him home with a sacrifice fly, and Juan Soto knotted the score with a solo homer to left-center two batters later. Then Matt Adams strode into the box and made it back-to-back off Diamondbacks starter Taylor Clarke.

Adams’s solo shot, a towering drive to the second deck in right, came just five days after the Nationals thought he may go on the injured list. Adams felt pain in his oblique while checking his swing against the Chicago White Sox at the start of the week. Washington considered shutting him down then — even flying infielder Adrián Sanchez from Class AA Harrisburg to meet the team in Chicago — but decided to gamble with a short bench and see how quickly Adams recovered. He woke up the next day feeling much better and, as it turned out, the supposed oblique sprain was just back spasms.

That led him back into the lineup Saturday, and it meant he didn’t have to sit for 10 days and further thin Washington’s depth at first base. But the first-inning blast, his seventh of the year, didn’t lead to a sustainable advantage. Strasburg gave it up right away, with no outs in the second, when Christian Walker lifted a middle-in curveball out of the park to center. Nick Ahmed followed with a triple, missing a homer by a few feet, and Carson Kelly singled him in to swing the score back in Arizona’s favor.

Seventeen batters had come to the plate in the first inning and a half; five went yard, two smacked triples, and Kelly, breaking the mold, dribbled that single through the middle to drive in a run. Strasburg gave up another run in the third, on two singles and a double, and Marte ripped his second homer an inning later. The 30-year-old just didn’t have it, from his first offering on, and had just one scoreless inning when he exited after five.

His outing finished at 95 pitches, only eight of them inducing a swinging strike, and he had trouble locating his fastball throughout.

“I was flying open a little bit,” Strasburg said of why he couldn’t spot that pitch. “Sometimes you just don’t have good command. I didn’t get away with many today.”

Javy Guerra kept the Nationals within range by tossing two scoreless innings out of the bullpen. Kyle Barraclough entered for the eighth, the score already lopsided, and fans headed for the exits after he gave up a two-run homer. Another run scored before the inning ended. Trevor Rosenthal came in for the ninth, walked the first two batters he faced, gave up a hard-hit single and let the Diamondbacks scratch across one more run before he escaped the jam with a double play.

But the Nationals had long been buried in a pile of Strasburg’s mistakes, sneaking into his otherwise-strong season and sinking them before there was much of a chance to compete. This loss was as simple as that.

“When you fall behind or you miss your spots, you’re going to have those days,” Martinez said. “But let’s just chalk this one up because he’s been really, really good up until this point.”