Cardinals Rookie Zack Thompson Has a Quality Curveball

© Jeff Curry-USA TODAY Sports

The Learning and Developing a Pitch series is back for another season, and we’re once again hearing from pitchers on a notable weapon in their arsenal. Today’s installment features St. Louis Cardinals rookie left-hander Zack Thompson on his big-bending curveball.

Drafted 19th overall in 2019 out of the University of Kentucky, the 24-year-old Thompson made his major league debut on June 3, and he’s gone on to log a 3.31 ERA and a 4.05 FIP over 16-and-a-third innings. Working primarily out of the bullpen, he’s allowed 13 hits, issued five free passes, and fanned 13 batters. No. 9 on our newly-released St. Louis Cardinals Top Prospect list, Thompson has thrown his arguably-best-in-the-system curveball 32.8% of the time.


Zack Thompson: “Growing up, my dad was always protective of me throwing breaking balls, so I didn’t start throwing one until I was a junior in high school. That’s when we began messing around with a curveball. We started out duct-taping two tennis balls together — my high school pitching coach, Jason Dudley, came up with the idea — and I just kind of flipped those to get the shape. It’s actually a lot easier to get feedback off of that. That’s kind of how it got started for me, and I ran with it from there.

“As I got older, I obviously started refining it more. The shape has essentially stayed the same, although I did have to cut down a little bit on the movement. That happened in college. Honestly, it was just too big. It was also too slow. Cutting down on the movement, my command got better, and the pitch also got a little bit sharper.

“It hadn’t been [popping out of the hand]. Again, it was just too big to command. Basically, it was a guessing game as to where to start it. I was having to start it way out there, and try to guide it in. What I did was tweak the grip a little bit. It was originally more of a spike, but then I kind of slid up to where my fingertip meets up at the top knuckle. That’s where I started to get more command with it. It’s kind of how turned into the weapon that it is now.

Zack Thompson’s curveball grip.

“Velocity-wise, it progressed from the upper-60s into the low-70s, and then into the mid-70s. Now I can rip one off in the high-70s, but I can also drop one in in the low-70s if I want. Overall, it’s gotten a little harder and sharper, and I cut a couple of inches off. I try to tunnel it off my four-seam, but it’s still big enough that it’s not easy to tunnel.

“Along with being big, my curveball spins harder than any of my other stuff. It typically lives somewhere in the 2,800 to 2,900 [rpm] range. It’s a little Kershaw-esque. I’ve kind of tried to follow in his shoes a little bit.”

David Laurila grew up in Michigan's Upper Peninsula and now writes about baseball from his home in Cambridge, Mass. He authored the Prospectus Q&A series at Baseball Prospectus from December 2006-May 2011 before being claimed off waivers by FanGraphs. He can be followed on Twitter @DavidLaurilaQA.

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6 months ago

I’m trying to figure out what throwing two tennis balls duct-taped together looks like.