MESA, Ariz. -- There is no shortage of highlights for Mason Miller to bask in from what was a massively successful All-Star rookie campaign. Yet when the Athletics closer looks back on his 2024 season, his mind immediately recalls the moments of failure.

Times like the walk-off homer he surrendered to Josh Jung in early September at Globe Life Field or Miller's blown save against Seattle in his final outing of the season are what sticks with him even more than his flawless inning in the All-Star Game -- in which he struck out Shohei Ohtani and threw the fastest pitch in All-Star Game history -- or setting the A’s rookie record with 28 saves.

“My last outing in Seattle, I gave up the game-tying home run, so that was easy to have that one sit with me,” Miller said. “A lot of that stuff is burned into my head. I just remember it. I tried to take a couple of weeks away from baseball and clear my head, then hit the reset button and get back to work.”

It's difficult to imagine a first full season in the big leagues going much better than Miller’s 2024 campaign. The 26-year-old flamethrower showcased a fastball that averaged 100.9 mph and a wipeout slider while posting a 2.49 ERA in 55 relief appearances with 104 strikeouts across 65 innings. His 14.4 strikeouts per nine innings ranked as the highest among qualified Major League relievers, while his 2.3 fWAR was tied for third-highest in a year that also saw him earn a spot on the All-MLB Second Team.

Still, there is another level Miller is striving to reach. And while no season will ever go perfectly, Miller identified some flaws in lesser dominant outings -- particularly his two losses and three blown saves -- that he is working to address.

“I love the offseason because it’s a chance to compete against yourself after competing against other people all year,” Miller said. “For me, a lot of times I got burned was the long ball with somebody on base. The Texas one [on Sept. 1] is the one that really bothers me because I kind of lost my slider and everybody in the park knew it was going to be a fastball. I’m trying to stay on top of things and be consistent, even through the struggles.”

A noticeably large crowd formed on the backfields of Hohokam Stadium on Thursday morning to watch Miller’s first bullpen session of Spring Training. The onlookers included everyone from players to coaches and front office members, even A’s legend Dave Stewart, whom the club recently hired as a special assistant to player development.

In his own version of “easing back” into things, Miller fired several fastballs to catcher Shea Langeliers that clocked in the upper 90s.

“It was kind of funny,” Langeliers said. “Whatever the percentage he was trying, he was still hitting 96 [mph] in the bullpen. Just Mason things.”

For as stellar as Miller’s 2024 went, Langeliers also believes there is another gear Miller can reach, especially when you take into account that Miller had zero closing experience at any level of professional baseball prior to last season.

“I think he’s just going to have an even better understanding of what he’s trying to do,” Langeliers said. “He’s going to understand when he needs to bring 104 [mph] and when he doesn’t. … That just comes with time. He’s still growing in that role and understanding the hardest three outs in the game to get. He’s the guy we want on the mound. Anyone will tell you the sky is the limit for him. I think he can be even better.”

Miller will lead a bullpen mostly intact from last year that combined for a 3.83 ERA -- 13th-lowest in the Majors -- and received a boost this offseason with the signing of right-hander José Leclerc. If the A’s are going to meet their high expectations of contending for the playoffs in 2025, that relief unit will need to be relied on as a strength.

“I think it speaks to how we are approaching games this season,” Miller said of the A’s offseason additions. “There’s not going to be as many opportunities to get your feet wet. Every time you come in, hopefully, the game is close. Having a handful of guys you can turn to and trust is huge, not only for our group, but also for the starters. Getting us through nine innings when our offense is able to put up a couple of runs, we feel pretty good about holding that lead.”