She clearly appears to be female as she has obvious secondary female sexual characteristics - rounded face, wide hips, smaller waist, breasts, etc.. She appears to be a female who has inherited from her parents a tendency to be muscular and to build muscle easily, but this isn't a purely male characteristic.
Whereas Semenya is more clearly male, with no breasts, a square shaped male face, heavier thigh muscles in front of the femur and so on. Semenya is also on record as having testes (which produce very high levels male levels of testosterone) and therefore must have a Y chromosome and be male (because humans cannot make testes without being XY) whereas Serena shows no signs of being anything other than a muscular, rugged female, who is very strong compared to more slight female competitors.
What I'm saying is that you get males (XY) with feminine traits (because they didn't correctly develop as males) but not females with male traits, because all foetuses start off as female and then roughly half develop into males.