PITTSBURGH -- Pirates right-hander Mitch Keller passed the 100-inning mark in his Major League career in Sunday afternoon’s 4-1 loss to the Giants at PNC Park, and there are signs that he’s beginning to figure some things out at the Major League level.

Keller has been on a rollercoaster ride of starts this season, struggling in one, then turning it around in the next. Both he and the Bucs’ staff are working overtime to shift the scale to the latter kind of outcome as the former No. 1 prospect works to become an effective Major League pitcher.

“If I had the crystal-clear answer or the key to turn that,” general manager Ben Cherington said, “I would turn that lock really quickly.”

Since we don’t have a perfect answer to the question, “What will make Mitch Keller the ace the Pirates hope he can be?” let’s look at what has gone right so far this season when he’s showing encouraging signs as he did consistently with five-plus innings with two runs allowed against the Giants.

Working ahead

Keller’s start on Sunday began with a run of success that is key to his ability to be effective and efficient. Through the first 14 batters, Keller did not allow any count to run to 2-0, a stretch which ended when Brandon Crawford took two pitches in the fourth inning ahead of a single.

Keller went to his fastball four times for strikes in 1-0 counts, then he missed with a curveball to Crawford. Having that kind of a mentality of attacking the zone paired with at least average control of the righty’s stuff is a big recipe for success.

The only way to know what adjustments Major League hitting will call for, as Cherington talked about for a young pitcher before the game, is to throw strikes -- even when it leads to something like a seven-run outing against the Reds on Monday, when Keller issued only two walks in an 18-batter span with a 65% strike rate.

Then, when Keller can figure out what things he can tweak with his game plan, he can adjust and truly command the zone as he did on Sunday.

“I thought it was the best he's been all year, in terms of attacking and executing,” manager Derek Shelton said.

Slow it down

Among the things that Shelton said were the best of the season for Keller on Sunday: his changeup and his slider.

The changeup is a pitch Keller hasn’t thrown much this season, but he used it eight times against the Giants with a lefty-heavy top of the order. Keller used it three times against Mike Yastrzemski in the third inning before finishing him off with a fastball for a strikeout looking.

The slider he’s thrown a lot more, but to mixed success. Keller has allowed a .724 slugging percentage against, per Baseball Savant, though the expected slugging percentage against is a much more manageable .527.

A large part of its success has been about location. The hard-hit balls -- defined as those with an exit velocity of 95 or more mph -- against Keller’s slider in 2021 have been either middle-middle or hanging up. But on Sunday, Keller attacked the low-and-away corner of the strike zone against right-handed hitters to keep them off balance consistently.

“I feel like I was getting into really good spots here today and not leaving it over the heart of the zone like I had been in the past,” Keller said.

Minimizing long at-bats

When Keller had the shortest start of the season against the Royals on April 28, he also had one of his most taxing as he was unable to find the third strike or ball in play in some long at-bats. Kansas City’s offense worked three at-bats of at least eight pitches, including an RBI double by Jorge Soler and a walk by Hunter Dozier.

But on Sunday, Keller had only one at-bat that went eight pitches. In the fourth inning against Darin Ruf, Keller placed nearly every pitch right on the border, drawing five foul balls. He sealed the strikeout on the eighth offering, running a high four-seamer -- a pitch Keller has gradually begun to master -- after working low, low, low.

“We've seen in previous starts where he hasn't executed, or if he has executed it one time, he hasn't been able to go back to it,” Shelton said of Keller’s fastball. “Today, we saw him be able to go back to it.”