Osaka, who has a Japanese mother and Haitian father, said in a social media post: “Before I am an athlete, I am a Black woman”.
Her decision follows protests over the police shooting of Jacob Blake, a Black man, in the city of Kenosha, Wisconsin, on Sunday.
Earlier on Wednesday, Osaka had battled back from a set down to defeat Anett Kontaveit 4-6 6-2 7-5 and reach the Western & Southern Open semi-finals for the first time.
She trailed 2-0 in the second set, but stepped up her serving and cut down on unforced errors to reel off six straight games.
The final set looked headed for a tiebreaker when Kontaveit rushed out to a 40-15 lead while serving at 5-6.
But fourth seed Osaka fought back and completed the comeback when the Estonian sent a backhand wide on match point.
“For the first set and honestly a couple of games in the second I was just really being down on myself and super negative,” said Osaka.
“For me, I just didn’t want to, you know, if I had to lose a match, I didn’t want to lose a match on that note. Yeah, I just tried to be more positive and pump myself up.”
Next up for the twice Grand Slam champion was a meeting with Belgian Elise Mertens, until her withdrawal.
The Belgian had needed just over an hour to defeat qualifier Jessica Pegula 6-1 6-3 and reach her first Premier 5 semi-final.(Reuters/NAN)