我是昨天...的 answers the question: "When did you do it?" 你是什么时候做的？ 我昨天...了 is just a statement that you did something yesterday. It doesn't answer a particular question.

The two sentence emphasize diffience. 我昨天打网球了emphasize play tennis. 我是昨天打的网球emphasize yesterday.

the sentence 我昨天打网球了 puts emphasis on what you did yesterday, while 我是昨天打网球的 puts emphasis on the date when you played tennis, it can be translated into "It is yesterday when I played tennis". In oral Chinese, we use the former sentence more often.

简单来说，两个句子是一样的意思，不过语法正确是第一个句子，第2个句子语法错误，但是我们也能明白你的意思。

“我昨天打网球了”— 'I played tennis yesterday.' It is a common statement sentence. “我是昨天打网球的”— 'It is yesterday that I played tennis.' It is like an emphasizing sentence, and ‘昨天’is the emphasis in this sentence.