There is not anything like "free-tennis-court" especially in CH. The fact that there aren't any people there and the net is still fixed on the court doesn't mean that in peak of the season one can just bump in there without prior notice and play anytime one wishes. Here is the link to their web site with contact details for direct bookings:
http://www.stadt-zuerich.ch/ssd/de/i...e/hardhof.html