BALTIMORE -- On May 18, 2022, Mitch Keller took the mound at Wrigley Field as a reliever. Keller had transitioned to a hybrid role following a rough beginning, temporarily exiled from the Pirates' starting rotation. Keller went viral during the offseason, then teased and enticed during Spring Training, but when the games counted, Keller couldn’t deliver how he wanted.

So much can change in a year.

After tossing his first career shutout on Monday night, Keller penned his latest masterpiece as the Pirates defeated the Orioles, 4-0, on Sunday afternoon. On a picturesque Mother’s Day at Oriole Park at Camden Yards, Keller struck out a career-high 13 batters with no walks across seven scoreless innings -- the first time a Bucs pitcher has recorded that stat line in franchise history.

A year ago, he couldn’t have imagined being in this spot.

“I was just trying to have a job, honestly,” Keller said. “I was just trying to put something together so I could stay in this clubhouse with these guys and have a uniform. It’s pretty crazy.”

“From what I've seen in the past with him and how much different he is right now, kudos to the Pirates, and to him, for turning into that. That was elite stuff," added Orioles manager Brandon Hyde.

When Keller took the hill that afternoon in Chicago for his first relief appearance of the season, he was defined more by his potential and less by his production. The YouTube TV broadcast showed the now-infamous graphic that depicted Keller ranking last among qualified pitchers in several statistical categories, including win percentage, ERA, opponent batting average, WHIP and ERA+.

Across his first 203 Major League innings, Keller owned a 6.12 ERA. By Statcast percentile rankings from 2021 and '22, you can see why his ERA was elevated.

Across his past 183 innings, by contrast, Keller owns a 2.95 ERA and has been one of baseball’s best starters. His 2023 Statcast percentile rankings reflect this as well.

Much has changed over the past year. Keller added a sinker. He added a sweeper. He added a cutter. Keller has six pitches in his bag, mixing and matching based on what is and isn’t working on a given day.

On this afternoon, just about everything was working as Keller generated a career-high 39 called strikes plus whiffs. Appropriately enough, Keller generated 13 of those called strikes plus whiffs with his sinker, the pitch that he introduced to the world in Chicago. The first sinker that Keller threw in a game hit Willson Contreras. On Sunday, Keller used that same pitch to strike out four batters.

"He's lights out, man,” said Bryan Reynolds. “Just great to see, from when he first came up to now he's evolved to what he is now. It's almost like I don't even need a glove out in the outfield.”

There’s also been Keller’s transformation on the mental side. Keller has been lauded for his newly found confidence on the mound, and in his past two outings, he’s certainly embodied the mentality of an ace. On Monday against the Rockies, the Pirates entered play with a seven-game losing streak.

The Pirates had four consecutive losses heading into the series finale and they were in jeopardy of being swept for the third time in their past four series. On both occasions, the Bucs needed Keller to play the role of stopper. On both occasions, Keller provided in spades.

“We’ve talked about the fundamental adjustments and the mechanical adjustments or the pitch mix adjustments," said manager Derek Shelton. "The biggest thing that I personally see is the look in his eyes. Before, it was like, ‘Am I going to execute a pitch?’ Now, it’s, ‘I am going to execute a pitch.’ I think the mental maturity there is a really huge step.”

So much has changed since that overcast day in the Windy City. Different pitches. Different mentality. Different results. Is this the same Mitch Keller?

"Hell no,” said pitching coach Oscar Marin. “He's not the same. It took that rest of the year for him to be able to transform into what he's done."

If Keller's not the same guy, then what is he now?

“He’s an ace,” Marin smiled.

This past week, Keller has publicly made it known that he would love to continue being the Pirates’ ace for years to come.

Following his complete-game shutout against the Rockies, Keller said on 93.7 The Fan and revealed that he and the team have discussed a contract extension, noting that there’s been “nothing too extravagant just yet.” While Keller noted postgame that talks have slowed down since the beginning of the season, general manager Ben Cherington said on The Fan that Keller checks enough boxes to warrant a potential extension.

“I’ve only been with Pittsburgh, so it’s all I really know,” Keller said. “I’d love to spend my whole career here, hopefully as long as possible. Having guys like [Ke’Bryan Hayes] and [Bryan] Reynolds locked up, it’s really cool to see some core guys getting locked in. I would love to be a part of that.”