Cutting already installed tile?
You're going to need two tools to do this and two specialty blades. A diamond cut-off disc to make the majority of the cut attached to a grinder. And you'll need a diamond (if available) or carbide-tipped oscillator bit and the corresponding oscillating tool ( sometimes called a multi-tool). For the majority of the cut you will use that diamond cut off disc with a wet sponge, a very wet sponge touching the blade obviously being very careful. Due to the diameter of the diamond disc you can't get right into the corners that's where the oscillating tool comes in handy. The oscillator will allow you to complete the cut right to the edge and then you can remove the tiles as required.

If you don't own these tools they should be available for rent but as a do-it-yourselfer both an oscillator and a grinder are essential.