(Patrick McDermott/GETTY IMAGES)

That, he told himself, wouldn’t happen this season. Mattheus, 28, improved his arm conditioning and lifting routines and, overall, now knew exactly how to manage the stress of a major league relieving job.

“Not that I wasn’t healthy last year,” he said. “I just didn’t really know what the rigors of a big league season. The minor leagues, as relief pitcher, they kind of monitor you. You don’t go back-to-back days very often. If you do, you get a couple days off.”

This season, Mattheus has logged more innings and done even better than his rookie year. He sports the third-lowest ERA (2.47) in the Nationals bullpen — behind Sean Burnett (1.94 ERA) and Michael Gonzalez (2.25), who has nearly half the innings as Mattheus. His numbers could be even better had he not missed 22 games in late May and early June because of a foot injury and one bad outing in Milwaukee in late July.

Saturday night was one of Mattheus’s most dominating performances of the season. He faced five batters over 1 2/3 innings, inducing two groundouts and striking out three batters, all of them looking. His fastball command was supremely sharp, his sinker utterly inviting.

In 11 innings over 10 appearances in August, Mattheus has allowed only two hits and no runs, walking only three batters and striking out six batters. More telling: he has induced 19 groundouts already, his highest total of any month this season and there’s two weeks left. That comes as a result of Mattheus learning how to use his biting sinker more effectively

He listened to catcher Jesus Flores tell him to trust his sinker more. Mattheus noticed that he had fallen into a pattern of throwing the sinker too often. He watched as another sinker baller, Ross Detwiler, learned how to mix in his sinker better with his fastball and where to place it.

“I was like, ‘That’s what I need to do,’ ” Mattheus said. “Then I started doing it. Then when [catcher] Kurt [Suzuki] got here, he was like, ‘Hey, if you’re going to do this, throw the four-seamer away and then sets up that sinker in.’ I started doing it, not so much away from throwing the split finger and the slider, just more little pitch over here, the four-seamer away. Then left-handers were able to dive on that sinker I throw 90 percent of the time.”

On Saturday night, Mattheus said it all clicked. A dipping 94-mph sinker inside looks inviting after a 95-mph fastball on the outer half of the plate. He threw only four sliders in all.

“It’s a process,” he said. “I’ve learned a lot more from last year.”

Gonzalez notched the final out of the eighth inning because Nationals Manager Davey Johnson wanted to get him some work, but he did so reluctantly.

“Mattheus was throwing the ball so good I didn’t want to go out there,” he said. “He’s been good all year, but he was special tonight, too. Extra special.”