PITTSBURGH -- Mitch Keller couldn’t hide his smile if he tried.

Keller’s final start was solid -- not his best, but an outing that put his team in a position to win. Across five-plus innings, Keller allowed two runs, both of which came by way of Albert Pujols’s 703rd career home run, as the Pirates beat the Cardinals, 3-2, on Monday at PNC Park. On other nights, Keller may have put himself under a microscope. On this evening, though, Keller took a step back and basked in his body of work.

In his fourth season, Keller found not only the success that has eluded him in previous years, but the confidence that comes with it. He posted several career bests. Over six months, Keller gradually lifted the burden of underperformance from his shoulders and composed the best full season of his young career.

“It’s always really nice to have a successful season, and I think I’ve done just that,” Keller said. “To do it at the Major League level is awesome, especially with it being my first time I’ve really put it all together. It’s just something I can grow on and keep going.”

This season certainly qualifies as the foundation upon which he can build. Across a career-high 159 innings, Keller posted a 3.91 ERA (his best in a full season, not counting his 2.91 ERA in 2020, when he only pitched in five games) and a 3.88 FIP (his best in a full season, not counting his 3.19 FIP in 2019 over 11 games) with a career-high 138 strikeouts. Keller’s fWAR and bWAR have yet to be updated, but those numbers will be career highs as well. These last six months, of course, weren’t all smooth sailing.

For all the strides Keller made in the offseason, Keller had a 6.62 ERA by the end of April. In mid-May, Keller found himself in the bullpen. Keller never saw the move as permanent, but he said that he was “going through it a little bit.”

On May 18, the day he made his first relief appearance of the year, Keller had his watershed moment. It was on this day that Keller unleashed his sinker, a pitch he had never thrown in a Major League game, to the world.

He only threw one that evening and it hit Willson Contreras, but Keller had found the pitch that would define his repertoire.

In his next outing, a relief appearance against the Rockies, Keller threw 19 sinkers. Upon re-joining the rotation May 31 against the Dodgers in Los Angeles, Keller threw 34 sinkers, the first time he’d thrown it more than any other pitch. Keller was now a sinkerballer.

The sinker, as well as the introduction of the sweeper, played an invaluable role in Keller’s development as a pitcher, but that pitch was just one reason for his success, not the whole reason. There’s also the mental side of things.

During the year, Keller found himself in several tough, back-up-against-the-wall situations while on the mound -- situations that he would not have been mentally equipped to handle in the past. This season, however, Keller displayed a refined mental fortitude. On a plethora of occasions, he bore down, escaped trouble and put the Pirates in a position to win.

There were several outings in which Keller showcased this resiliency, his start against the Mets on Sept. 16 stands out. In the bottom of the sixth, the Pirates already trailing, 3-1, the Mets loaded the bases on Keller. One swing could upend the game.

Manager Derek Shelton came out to the mound, but didn’t immediately signal to the bullpen. Shelton wanted to hear Keller’s plan of attack. Keller laid out exactly how he wanted to attack Luis Guillorme, the exact response that Shelton wanted. Keller remained in the game -- Shelton said postgame that Keller earned that trust -- and retired Guillorme on a very softly hit line drive.

Keller admitted that, in the past, he might have been looking toward the dugout, looking for someone to take him out of the game. On that night, Keller had the confidence to finish what he started. Keller didn’t just grow on a fundamental level, but on a mental level.

“The number one thing that’s jumped out to me is demeanor,” Shelton said. “I think every time he takes the mound, he expects good things to happen. That’s something you have to develop at the Major League level. I think Mitch, in the past, has been in situations where he may have doubted that something good was going to happen. Now, he really thinks, ‘All right, I’m in control. I’m going to go after people.’ That’s definitely a sign of growth. It’s a sign of maturity. It’s a sign of a young player coming into himself, and I’m really proud of him for that because that not only takes fundamentals, but that takes mental fortitude to get there. I think we’ve really seen him grow that way this year.”

The work is not done for Keller. After rediscovering his velocity last offseason, Keller said he plans to make additional tweaks to his sinker, as well as potentially add a “gyro” slider in addition to the sweeper. That work will be done in due time, but with two games remaining, Keller can take the opportunity to reflect on the steps that he has taken, on the evolution he underwent.