This is very well done. I like the imperfect, cute and unique art style. The mechanics work great and it fits the theme with the never ending tennis balls. I also think the little boing sound effects fit the game extremely well. The normal play mode is also good to learn the game before trying the hard mode which imo should be the main mode.
The only thing I think could be improved is the volume of the music. The music is extremely quiet compared to the sound effects but that's honestly just nitpicking. Great job :)