Naomi Osaka had a clear message throughout her championship U.S. Open run in New York: Black Lives Matter.

Before her first match and all the way through until her victory over Victoria Azarenka in the Final, Osaka came to the court in a customized mask that featured the name of a Black person who had died at the hands of police violence. She wore masks with the names of Breonna Taylor, Elijah McClain, Ahmaud Arbery, Trayvon Martin, George Floyd, Philando Castile and Tamir Rice.

The gesture was one of protest and of solidarity, something she faced a lot of heat for...