Race was the most important aspect of individual identity in apartheid South Africa. It determined where you lived, who you married, and what kinds of education, job, and housing was available to you. Whites were the privileged elite, with access to the best education, lucrative jobs, and the ability to employ black servants at non-living wages. On the other hand, blacks were systematically oppressed at every turn; their lives were controlled by an unsympathetic government that saw them as inferior beings. The majority of social, legal, educational, political, and religious organizations worked to keep the apartheid system in place and to prevent blacks from escaping poverty and ignorance and gaining a position of equality with whites. Under apartheid, whites banded together to keep blacks oppressed, while blacks were splintered by their ethnic identities and indigenous languages, a practice that government officials exploited and encouraged.
It wasn't until the Soweto school riots in 1976, which Mark describes in detail, when blacks began to unite on a wide scale and fight against their common oppressor. (We learn after the fact that this was never a unified movement, as the police quickly moved in to incite enmity between various groups and tribes.) We do see how Mark suffers ostracism after he decides to strike out on his own and play tennis after black tennis players turned against him. Mark decided that he needed to look out for his best interests rather than the collective interests of black athletes. This was a decision that made many fellow black athletes angry.
Even though Mark was terrified by white people as a child, he had several experiences that enabled him to work hard to feel comfortable within the white world.