Here's why:
* You can count tennis matches, balls, courts, players, etc. For example, "We played three tennis matches today."
* You can use the plural form "tennis matches" or "tennis balls".
* You can use numbers and articles with it. For example, "I bought a new tennis racket" or "There were five tennis players on the court."
Uncountable nouns, on the other hand, refer to things that cannot be easily counted or separated into individual units. Examples include water, advice, or information.
Therefore, tennis is definitely a countable noun because you can count its occurrences and use it in the plural form.