Masahiro Tanaka is currently making a strong case to be the Yankees American League Wild Card starter. Given his performance before the all-star break, that would seem impossible to imagine. His ERA was 4.54 with a FIP of 4.90, to go along with 18 home runs, good for a 1.94 HR/9. That is back-end rotation numbers or even enough to get someone pulled from the rotation.

But the second half has been a 180-degree turn for Tanaka on all accounts. His HR/9 has slashed to 0.77, his ERA is 2.30 with a FIP of 2.79. His K% has increased from 24.3% to 26.5% while his BB% has dropped from 6.5% to 4.4%. Tanaka has adjusted since the all-star break, but if you look at the last five years, he is usually better during the second half.

His ERA in the first half since 2014 is 3.77 wherein the second half it is 3.23, with a FIP dropping from 3.91 in the first half to 3.62 in the second half. For now, the focus is on 2018 and what has changed for Tanaka.

To help understand the decrease in ERA, it is helpful to look back at the HR/9 along with his HR/FB rate in the first and second half. This is because Tanaka does a pretty good job of limiting the walks but has consistently struggled at preventing home runs. Part of that can be attributed to being in Yankee stadium, but these numbers suggest that’s not the only issue.

For one, Tanaka ran a HR/9 rate of 1.94, with a HR/FB (home run per flyball rate) of 20.9%. League average sits around 10-11% for HR/FB. Career-wise, Tanaka has a HR/FB of 17.8% but in the second half, he has cut it down to 11.6%. Homerun prevention is partially luck, but there is one thing that pitchers can do to prevent home runs and that is reduce the number of flyballs they give up.

Tanaka has done just that, with his FB% (flyball rate) from the first half at 38.1% being reduced to 27.6%. The fewer flyballs, the fewer opportunities for them to turn into home runs. What goes hand in hand with that is his GB% (groundball rate). He has spiked it from 38.1% to 51.9%. Groundballs, next to strikeouts, are a pitcher’s best friend as they generally do not generate many runs or extra bases. That is not to say they do not go for hits, but limiting extra bases is important.

For this analysis, the only pitches that will be looked at are Tanaka’s fastball, slider, and splitter as they account for nearly 95% of the pitches he throws.

First, has his pitch mix changed at all? Slightly! In a small sample of September, Tanaka has upped his fastball usage, which is not his best pitch but has maintained the same amount of usage of his splitter and slider. See below:

The thing that should be a focus is the kind of contact Tanaka has given up. Below are two charts showing the flyballs per ball in play and groundballs per ball in play, split by pitch type:

There has been a leveling out and an overall decrease in the number of flyballs per fastball in play. Meanwhile, we have the inverse for groundballs in play with the fastball. Slider and splitter have mostly stayed steady. What could be the reason for more groundballs and fewer flyballs? Here’s a heat map of pitch location from the first half and the second half respectively:

The difference is nuanced but specifically look at the bottom part of the heat map. There’s slightly more purple in the middle of the zone during the first half, wherein the second half that purple has shifted to blue and the bottom half has become a stronger red. Tanaka is doing a better job of stay low in the zone and it’s helping slash his home runs, which were the huge problem in the first half.

Here are two gifs from August 27th and September 1st, respectively. They are from Rob Friedman on Twitter (follow him, the gifs are great @PitchingNinja) that demonstrate his change in approach.

Masahiro Tanaka, 95mph Fastball and 89mph Splitter, Overlay. pic.twitter.com/wUCgc3P6Vz

— Rob Friedman (@PitchingNinja) September 1, 2018

Masahiro Tanaka, 89mph Splitter and 87mph Slider, Overlay.

😮 pic.twitter.com/ljjmhj7Mw6

— Rob Friedman (@PitchingNinja) August 28, 2018

By staying low and increasing the number of groundballs and reducing the flyballs, Tanaka reduces the number of fly balls that turn into home runs. Combine this with his ability to limit the walks and a stellar strikeout rate, and it is obvious to see that Tanaka is hitting his stride at the perfect time. He looks to continue this success on Friday night.

Stats are through 9/10/2018 and courtesy of FanGraphs, charts are courtesy of BrooksBaseball