MESA, Ariz. -- In 2018, Mason Miller was a Division III pitcher at Waynesburg (Pa.) University who posted a 7.16 ERA. Fast forward to 2022, and Miller has become something of a prospect sensation during the Arizona Fall League with his eye-popping radar gun readings.

Miller, now 24 with just 20 innings of pro ball under his belt, is the Athletics’ No. 20 prospect. He has routinely ripped off triple-digit fastballs when healthy, and he did so again with frequency during Mesa’s 7-6 loss to Scottsdale at Sloan Park on Tuesday, topping out at 101 mph. He retired all nine batters he faced, striking out five of them.

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How has this journey been possible?

“It’s happened fast, for sure,” Miller said. “It’s like every year I look up and something awesome has happened to me.

“I don’t take anything that’s happened for granted. I’m definitely lucky to be in the shoes that I’m in. Now it’s just making the most of it.”

Diagnosed with Type 1 juvenile diabetes in 2018, Miller battled his way from 150 pounds to become the 200-pound hurler he is with Mesa. Transferring to Gardner-Webb University for his senior season in ‘21, he whiffed 121 batters and compiled a 3.30 ERA in 92 2/3 innings, which set the stage for Oakland to select him in the third round of the 2021 MLB Draft.

Each time Miller ripped off a fastball Tuesday, an audible murmur emanated from the crowd. His heater consistently sat at 99-100 mph, and he deployed it to finish off three batters, drawing the oohs and aahs of those in attendance.

“With this level of hitters, I can blow one by guys, and then they can be right on the next pitch,” Miller said of his fastball. “It can still be overpowering, of course, but picking my spots with it is a lot more effective.”

Miller peppered in 22 of his 29 pitches for strikes. While his fastball grades out as his premier weapon according to MLB Pipeline (70, on a 20-80 scale), his secondary pitches -- a slider and a changeup -- are largely his keys to success.

The changeup in particular hangs in the balance as the likely deciding factor between whether Miller can continue to hone his arsenal in the rotation. Two of his punchouts vs. the Scorpions came against the offering.

“I think moving forward, it’s the key for me to remain a starter,” Miller said of his changeup. “Just getting guys off my fastball, weak contact, early outs -- that’s kind of the goal behind it.”

Through five starts on the fall circuit, Miller has been largely dominant. Squaring up the 6-foot-5 righty has proven fruitless, as opposing batters have delivered just a .152 average against.

A right scapula strain kept Miller from making his season debut this year until Aug. 23. Over his six starts across three levels in 2022, he never exceeded 43 pitches, with Oakland exercising extreme caution on an arm that has proven potent. He rose as high as Triple-A Las Vegas for his final two outings, showcasing that while he lacks experience, his pure stuff has him knocking on the door to big leagues. Even with veritable training wheels on, the flamethrower struck out half of all batters he faced this season.

“This year has been a tough year, for sure,” Miller said. “But finishing it healthy has been great. Finishing it on a high note performance-wise is even more uplifting for me.”

Fellow A’s prospect Lawrence Butler (No. 14) enjoyed a two-hit, two-RBI performance while serving as Mesa's designated hitter. He opened the scoring for the Solar Sox with an opposite-field knock in the second, adding another single to left to plate a run in the eighth. The lefty batter raised his OPS to .866 across 16 games in the Fall League.