"Tennis is tiring."
No, が is not appropriate. By looking at the English translation, one may think that tennis is the subject of the Japanese sentence, however it is not. The subject is "I" and tennis is just a topic.
疲れる translates to "tired," never "tiring." The full sentence is 私はテニスをすると疲れます "I get tired when playing tennis." This is called a Konjac sentence こんにゃく文 which is described here => https://www.japanallover.com/2017/07/unagi-and-konnyaku-sentences-in-japanese/