BCN Ranking published their latest full-frame mirrorless camera market data for the Japanese market. In the first graph, you can how Sony's nearly 100% market share fell down to 67% after the announcement of the Nikon Z7 and Canon EOS R (Sony in blue: 67%, Nikon in green: 10.4%, Canon in red: 22.1%):
The second graph shows the distribution by sensor size: blue: APS-C, red: MFT, green: full-frame, purple: other (the graphs are for October 2017 vs October 2018):
As far as I know, BCN+R collects sales data from approximately 2/3 of all Japanese retail stores.
Via NikonRumors