Here you can find the current version of Pixel Tennis client. Please note, that the game is currently in a testing phase where the game server is only online at certain time spans. If you get a “Could not connect to game server”, this most likely means, that you tried to access the server outside of its available hours. See the News for the schedules.

Current version: 0.2.3 (22.06.2020)

When you are executing the installer, you might get a warning with the title “Windows protected your PC”. This is completely normal, because Windows treats downloaded unknown executables as malicious. As soon as more people download it, this annoying “security” message will not appear anymore - at least until the next version is released...
You should see an option to run the file anyway.