In tennis your opponent is half the match

You don't need to gather four, you need to get one right. A match against someone who walks all over you — or the reverse — lasts twenty minutes and nobody asks for a rematch, so finding the right person is literally the sport.

What makes a match happen twice

Level, stated plainly
Nobody gains anything by overselling or underselling: the match lasts as long as it lasts and it shows in the first game. A level written honestly is what makes the second match exist.
One spot and that's it
A tennis Nid fills with one person. It's the simplest format here, which is why it spends the least time open.