system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do professional tennis players always take a spare ball when serving, despite having a small army of ball boys/girls to serve them new balls on command?

...surely running around with an extra ball in your pants is uncomfortable and puts you at a disadvantage to your non-serving opponent?
gpt
For each point, they get two chances to serve.  They get a "do-over" if the serve hits the net or goes outside the lines.

Getting a new ball for every serve would just slow the game down too much.  Any disadvantage is negated by the fact that the other player does it too.