Suppose that Albrecht is a jerk. He comes home to his wife Gertha and proceeds to order her around. He expects her to cater to his desires, and when she does not he admonishes her in cutting tones.
Many would say that Albrecht is “selfish” — using “selfish” to mean someone who expects benefits at others’ costs. The “selfish” person puts his own interests above the other’s interests and acts to achieve a win/lose result.
Now suppose that Gertha is self-effacing. When Albrecht comes home, she willingly subordinates herself to him. She asks only what he wants, and she strictly admonishes herself not to count the personal costs.
Many would say that Gertha is “altruistic” — using “altruist” to mean someone who selflessly serves the interest of others. (And that is a standard dictionary definition: “the principle or practice of unselfish concern for or devotion to the welfare of others” — here, here, and here.) The altruist puts others’ interests above their own and acts to achieve a lose/win result.
Note that neither is “pro-social” in any healthy sense. Pro-social principles foster the well-being of both/all parties in social relationships. That is, they intend and act for win-win results.
The implication of that is that both “selfish” and “altruist” are equally anti-social. Just as Albrecht’s being a jerk is repulsive, so is Gertha’s being self-effacing. Healthy society respects individuals to assert their legitimate self-interests and to strive for mutually beneficial, win-win results.
I think this applies to sports competitions, like racquet sports. You strive to win but you spare your opponents complete humiliation and vice versa.