Felipe Rivero, here throwing batting practice during spring training, didn’t see himself as a reliever until the Nats moved him last season. Now, with a fastball that touches 100, he could be a future closer. (John McDonnell/The Washington Post)

When Washington Nationals fireballing left-handed reliever Felipe Rivero hit 99 mph against Baltimore Orioles slugger Chris Davis in an otherwise meaningless Sept. 24 game, he stopped himself. He had been inching toward the goal he set in the offseason following an injured 2014 as a starting pitcher. Rivero’s velocity improved each month as he built strength following an unexpected stomach injury, topping out at 97 mph in June, 98 in July and then 99 in August.

“ ‘Let’s see if I can do this,’ ” Rivero thought to himself. “’You never know when you’re going to pitch again.’ That day I felt really good. I had a feeling I could do it.”

As he had hoped, Rivero’s next pitch was 100 mph, a ball maybe two inches off the top inside corner. Even though he struck out Davis looking on the following pitch, a slider, Rivero’s triple-digit throw meant the most to him, not just because of the radar gun reading but because it symbolized how far he had come so quickly.

Rivero, 24, began his professional career as a starter and hadn’t dreamed of doing anything else until last spring, when the Nationals felt he could be more useful as a reliever. The Venezuelan native went from a hard-throwing mid-level left-handed prospect with shaky command to arguably the Nationals’ best reliever in 2015 and, perhaps, their future closer.

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“I trusted in myself, and I knew my arm had the potential to do throw 100,” Rivero said. “This year, I don’t want to stay there. I want to go up a little bit more.”

Acquired as part of the Jose Lobaton trade before the 2014 season, Rivero surprised himself in many ways last season: He didn’t think he would like relieving so much and do so well, he wasn’t sure how his elbow would respond after inflammation limited him to 14 minor league starts in 2014, and he wasn’t sure whether his left arm, which could hit 96 mph when starting but tired out easily, could throw harder in shorter relief stints. But after a season with a 2.79 ERA over 49 games, Rivero doesn’t want to go back.

“I feel like now relieving is more what I want to do,” he said. “I feel more comfortable coming out of the bullpen than starting.”

With a lanky 6-foot-2 frame, Rivero said he has always been blessed with a lively arm. In relieving, he fixed two of his biggest flaws from the minors. His arm didn’t tire in the fifth or sixth inning of a start, and he could throw harder. Last season, Rivero also cut down his walk rate; he was forced to be more aggressive because relievers have less margin for error.

“I was thinking too much before,” he said. “Last season, I’d imagine that, even if I was facing Barry Bonds, I’d get him out. Or when I threw against the league’s best batters, I didn’t think about the Mets or whoever. It’s me versus you. If I strike you out, I strike you out. If you make contact, you make contact. That’s it.”

That approach helped Rivero rise from long reliever to left-handed specialist to setup man and more. By the final week of a disappointing season, the Nationals were without suspended Jonathan Papelbon and injured Drew Storen. So on Oct. 1, Rivero was inserted in a save situation. He handled it well, notching a clean two-inning save. Two days later, he struck out the side on 11 dominant pitches for his second save. He loved the experience.

“Wow,” he said. “My first year in the big leagues, and I came from practically the bottom. The process happened so fast.”

Rivero said he will do whatever the Nationals ask of him in 2016. He is earnest when he says he just wants to pitch in the big leagues and win. And he is likely to serve as a setup man or left-handed long reliever. But Rivero, like most relievers, wants the most coveted job in the bullpen.

“If they give me the opportunity to close, I’ll gladly accept it,” he said. “Ninth inning, every reliever wants that. There are few lefties in the game, especially that throw like me, and that throw hard, there are much fewer. I’d love to be a closer of the Nationals or whatever team one day in the future.”

There’s a chance Rivero’s role could expand by 2017. Papelbon, 35, will be a free agent at the end of the season. And because relieving can be such a volatile job, building a bullpen from within is more sustainable. Rivero is under team control until 2021. Shawn Kelley, another option with high-strikeout stuff, has four career saves and is signed through 2018.

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“Felipe has the qualities to become a closer at this level,” catcher Wilson Ramos said. “He throws hard, has good pitches and is aggressive. He can be a closer. He doesn’t need much more to do it in the big leagues.”

Rivero hit 100 only once last season but thinks his arm can do it more consistently this year — and perhaps more.

“It’s a gift that God gave me,” he said. “I think it’s worth taking advantage of it while I can. We’ll see.”