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What happens after a rikishi wins 10 matches in a basho?

This seems elemental to understanding sumo. If a double-digit victory performance provides momentum, that is key to knowing who will do well. Similarly, a drop off after a massive promotion, which 10 wins usually betokens, is a sumo trope. Going from Maegashira 12 to Maegashira 3 (exactly what Aoiyama did between Haru and Natsu) is not usually a sign of performing well.

It is probably worth it to actually figure that out, though. Especially if you are playing a fantasy sumo game. In order to see how we might be able to predict the future, I looked at every 10 win performance since the Hatsu 2018 basho, and their subsequent performance. You can see the full list here. That boundary was picked to provide three full years of sumo. Of course, there's been tremendous upheaval in that time as well. Multiple Yokozuna have retired, while other rikishi have been promoted and demoted from Ozeki. There was also a missing basho for Natsu 2020, and a huge number of rikishi missed out on Hatsu 2021, both because of coronavirus issues.

Yet this selection of basho also featured a huge number of rikishi who got 10 wins and even yusho seemingly out of nowhere. With missing Yokozuna and a slow, odd changing of the guard, this period was a time when it seemed almost anyone could challenge for the yusho. And if you are looking at playing a game where it's important to know who's going to win a lot in a specific basho, it's good to know if 10 wins means you're likely to win 10 more.

Let's start by saying it is decidedly unlikely. In this time period, 49 rikishi have achieved 10 wins on 137 separate occasions. Just 18 times did a Rikishi win more than in their previous basho, and only 32 times did a rikishi follow up a 10 win basho with another 10 win basho. On average, rikishi lost a little less than 4 more times in their follow up basho.

There is a massive caveat that should be mentioned here. 13 times a rikishi missed all or nearly all of a basho after garnering 10 wins because of injury. These are mostly Yokozuna or Ozeki, along with Chiyonokuni being out for January 2021 due to his entire stable missing for COVID reasons. Even without those special cases, the average fall off was 3 wins. If a sumotori gets 10 wins, then loses 3 more times, they will get a losing record.

There's an obvious answer here. Many rikishi who get 10 wins will then see a big promotion. A big promotion means a step up in strength of schedule. Then they lose. 48 rikishi ranked Maegashira 10 or lower have collected 10 wins in the time considered. On average, they lost a little more than 7 more matches in their next tournament. The standout example of this group is probably Tokushoryu. After his miracle yusho in January 2020, he went from Maegashira 17 to Maegashira 4. And he went from 14 wins to 4 wins. Last basho's pleasant surprise is probably in for a rude awakening.

So who does gain wins? Healthy Sanyaku rikishi. On the 18 occasions a rikishi gained wins after double-digit victories, just 5 were Maegashira. Asanoyama in December 2019, Shodai in January 2020, Mitakeumi in May 2020, Wakatakakage in November 2020, and Daieisho in January 2021 (a yusho). The only one on that list who was neither previously a Sanyaku wrestler or on his way is Wakatakakage, and he is extremely close.

Back to back 10 win performances is the mark of a truly excellent rikishi. It should never be counted on or expected. Surpassing a previous win total is rare. Even Terunofuji, the only rikishi on this list who has averaged more than 10 wins in the followup basho, fell off somewhat. He averaged over 12 in the "first basho."

So if you want to put your faith in Aoiyama, Tobizaru, or Hidenoumi to repeat their excellence from March, be warned. They are likely to fall off hard.