I currently own a property in Edinburgh, UK with what used to be a tennis court. It is depicted here:
What would be the cheapest way to convert this back into a tennis court?
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Pressure-wash away the moss.
Assess the integrity of the concrete. If it's sound, carry on. If not, abandon the project as there's no "cheapest" way of restoring it.
Acid etch the concrete to make it completely clean for recoating.
Skim any divots with vinyl repair compound to make them flat.
Commission a court finisher to apply the appropriate court coating and paint the lines. Alternatively, roll or spray a suitable court paint yourself.
Disclaimer: I've never restored a tennis court, nor have I perfected the overhand serve. Or even tried. Either thing.