Jayson Werth enjoys his first four-hit game as a National and just his third game with at least three hits and two RBIs in Washington. (Nick Laham/Getty Images)

Tuesday night, under a starless sky at Citi Field, Ross Detwiler pitched exactly how the Washington Nationals envisioned he would in 2007, when they drafted a beanpole left-hander out of Missouri State with the sixth pick. Five years later and one week after the Nationals made him part of their rotation to start the season, Detwiler had arrived.

With five overpowering innings in a 6-2 Nationals victory, Detwiler handed the New York Mets their first loss and validated the Nationals’ decision to include him in their initial starting rotation for the first time.

Detwiler tossed five scoreless innings, allowing two hits and a walk and striking out six, retiring the final 10 hitters he faced and letting just three balls escape the infield. He spotted his low-90s sinker in every corner of the strike zone and bewildered Mets hitters with his vicious curveball.

Detwiler had pitched in the majors every year since 2009, toggling between the majors and minors, the rotation and the bullpen, healthy and infirm. But he had never pitched so close to opening day, never quite like this.

“The last few years, I’ve been kind of up and down, changing teams,” Detwiler said. “Before, I was just filling. Now, especially being on the opening day roster and being a starter, I feel like I belong here a little more.”

Because Detwiler worked as a reliever this spring and had not been stretched out, he lasted only 71 pitches over five innings. But during his time on the mound, he devastated the Mets.

The first two batters he faced knocked hits off him, a double by Ruben Tejada and a single by Ronny Cedeno. Detwiler retired 15 of the next 16 hitters, yielding only a walk and allowing one ball out of the infield.

The Nationals (3-2) retooled their starting pitching this winter, adding two high-priced acquisitions in Edwin Jackson and Gio Gonzalez and welcoming back Stephen Strasburg. No one would have guessed the best performance the first time through would come from Detwiler, the 26-year-old who had been there all along.

“He’s competing and he’s confident,” Nationals pitching coach Steve McCatty said, sitting in the dugout before the game. “Before, you’re competing and you’re not really sure what you’re doing. Conviction might be a better word. He knows what he wants to do. He’s doing a much better job of that. He’s learning.”

Tuesday, Detwiler received offensive support from the game’s first batter when Ian Desmond crushed Mets starter Dillon Gee’s third pitch deep into the left field seats. Jayson Werth smacked four hits for the first time since June 2009 and drove in two runs. He produced that feat — at least three hits and multiple RBI in the same game — twice in all of 2011. Ryan Zimmerman notched the 500th RBI of his career with a sacrifice fly in the eighth inning.

“I just focused on being more relaxed up there,” Werth said. “Looking back, that’s what I’ve done. It’s been a focus. I felt like I’ve been swinging the bat pretty good, but haven’t had a whole lot to show for it.”

But, even though he pitched just five innings, the night belonged to Detwiler. Tejada ended his first at-bat by drilling a double off the left field fence. Cedeno followed with a single that sneaked through the hole on the left side, the kind of hit that once would have irritated Detwiler to the point of distraction.

After Detwiler’s miserable start at Class AAA Syracuse last year, Chiefs Manager Randy Knorr, now the Nationals’ bench coach, pulled Detwiler into his office. Knorr told him, in no uncertain terms, that he had looked defeated on the mound, that he needed to fix his attitude.

”From the next start on, I kind of had a different demeanor about pitching,” Detwiler said. “Every pitch. It kind of molded it right there. That was the one point I can always look back on to say, ‘This is when everything changed.’ ”

And so, Tuesday night, with men on the corners and no outs in the first inning, Detwiler bore down. He struck out Daniel Murphy swinging at a high, 92-mph fastball. Jason Bay popped up to short. Lucas Duda whiffed at a 91-mph sinker at the knees.

The inning was over, and the Mets (4-1) never had another chance. Detwiler faced 13 more batters and stuck out four of them while inducing seven groundball outs. He froze the Mets with curveballs and challenged them with sinkers, unafraid to throw them high out of the zone and move them around every quadrant of the zone.

By throwing hard, four-seam fastballs inside during the early innings, Detwiler set up his sinker for later. Wilson Ramos caught Detwiler at the end of last year, but he had never seen Detwiler’s sinker break like it did Tuesday.

“That’s his best pitch right now,” Ramos said. “That was the key.”

When Detwiler returned to the majors late last season, McCatty noticed a different pitcher. Detwiler had regained strength in his right hip, which was surgically repaired in February 2010. He had fully committed to new mechanics meant to prevent further issues with his hip.

“He was back to what he showed when they drafted him,” McCatty said. “Better velocity. All that stuff was there. He was ready to pitch.”

Detwiler closed the year with 131 / 3 consecutive scoreless innings, which meant his streak grew to 181 / 3 Tuesday night. The Nationals anticipated evaluating Detwiler for their rotation this spring, but Detwiler didn’t know what to think. The team spent the winter bringing in starters and then put him in the bullpen as a long reliever during the spring.

“All the signings they made, it was kind of, ‘What’s going to happen with me?’ ” he said. “Am I going to go somewhere else? Am I going to go to the ’pen? What’s going to happen?”

Detwiler may have to wonder again soon. Chien-Ming Wang could return within three weeks, leaving Detwiler headed back to the bullpen. He will force the Nationals to find a spot for him if pitches like he did Tuesday, a night when Detwiler’s future seemed so very bright.