This works for me:
o Download and Install Karabiner from https://pqrs.org/osx/karabiner/
o Launch Karabiner, go to "Misc & Uninstall", find "Custom Setting", click on the "Open private.xml" button
o Use a text editor to edit the private.xml file found in the folder. Copy/Paste the following into the xml file then save:
<item>
<name>Change F19 to Command-Control-F</name>
<identifier>private.f19_to_command_control_F</identifier>
<autogen>
__KeyToKey__
KeyCode::F19,
KeyCode::F, ModifierFlag::COMMAND_L | ModifierFlag::CONTROL_L
</autogen>
</item>
o Back to Karabiner, go to "Change key", then click on Reload XML button.
o Check the checkbox "Change F19 to Command-Control-F" that newly appears.
This would allows switching between fullscreen and single window by only pressing F19.
You could redefine using other keys than F19. Note: macOS 10.13.2 seems not have been supported by Karabiner yet.