Tsukuba is a tennis paradise. There are many courts all around the city. The tennis courts on the grounds of various research institutes are almost never full and you don't need a reservation to use them. If you go by car, just register at the guard office. If you go by bike, there is no need to register at all.
This page shows all Tsukuba's tennis courts information (in Japanese). You can either walk-in if the court is available, or reserve it around 1 month in advance.
There is an international tennis club based in Tsukuba that offers tennis lessons in English. The members are half foreigners, half English-speaking Japanese.