What is correct - playing "on a squash court" or "in a squash court"?

Solution 1:

Use of prepositions in English is frequently idiomatic.

In your example "fish lives in a fish tank"-In tells us the noun is in an enclosed space (surround or closed off on all sides). Basically, when something is inside something.

• In a box
• In a room
• You probably couldn't get an aircraft carrier through there, but my kayak would have looked like a peanut in a squash court.

Consider the following:

All of the floor-markings on a squash court are only relevant during serves.

On tells us that the markings are located on a surface.

• On the table.
• On the floor.

Solution 2:

In is for containers, and contents of containers.
On is for surfaces, and locations with respect to surfaces.

If you think of it as playing in a container, use in.
If you think of it as playing on a surface, use on.

This gets asked quite a lot here. And answered.