She's not.
Osaka does have a large blonde streak, as shown in this photo:
https://image.~ibb.co~/iyPXX9/osaka_title_759.jpg
Knight has over-exaggerated this feature, giving the impression that Osaka's hair is completely blonde. So that's strike one.
But when I compared the skin colours for each person in the picture, I found nothing to support the claim that Osaka is shown to be white.
https://image.~ibb.co~/ekA6s9/41661049_10156415096611327_9169191780329979904_o.jpg
Top: Williams. Middle: Osaka. Bottom: Ramos. Osaka is very clearly brown here. This is so obvious, it's clear to the naked eye.
No one is disputing the shading used. Yes she is given a darker skin colour than the umpire, but that is irrelevant.
Both the umpire and the opponent are clearly not supposed to be the umpire that was there + Osaka because they look absolutely nothing alike. The artist has not even attempted to make them resemble the people that were actually there. If he did , he would have at least attempted to get the hair right for Osaka, or at least attempted to depict the umpire similarly.
I'm awful at drawing but it took me 2 seconds to actually make this look like Osaka, which the artist could have done if he wanted to:
https://s8.~postimg.cc~/p1c2ssi79/Osaka.jpg
Can you seriously, with a straight face tell me that if someone showed you this:
https://s8.~postimg.cc~/n9j3xw6k5/woman.jpg
..that you would think that was anything other than a caucasian woman with long blonde straight hair?
If you then looked at the picture of Serena, you would immediately ascertain that it was a drawing of a black woman. There is zero doubt in that.
The choice of the straight long blonde hair and the much lighter skin colour used (compared with Serena) unfortunately makes it look like he has just stuck two skinny caucasian people in the background as direct contrast to the oversized (and oddly fat looking) Serena caricature.
I will say again, i don't think it was intentional by the artist, but i find it odd how so many in this thread are refusing to see why the picture has been interpreted in the way that it has by some people