What will be the correct preposition to complete the sentence? 'I am not bad ____ tennis'.

• Bad at something:
English meaning: These words describe a person who lacks skill in doing something.

• The word with the broadest use is bad. You can be bad at something or bad at doing something. 
Example: We’re really bad at predicting future costs.

• দক্ষতা/অদক্ষতা বুঝাতে শব্দের সাথে at ব্যবহৃত হয়। যেমন- good at, bad at etc.

যেমনঃ
He is bad/good at cricket. [a bad/good player]
Running is bad/good for you. [unhealthy/healthy]
I am not bad at tennis.
He is good at chess.

Complete sentence: I am not bad at tennis.

Source: Cambridge Dictionary.