A question on quoting (people in news articles). [Details inside].

The mumble theory would be my guess. The player said something unclear, and the writer used the parentheses to indicate that that was what the writer thinks the player meant.

It is an odd construction and unclear what the writer intended. Either the writer put in parentheses what he thought Nadal meant but had omitted or what he thought Nadal had mumbled. Presumably, the journalist thought he meant “I hope I am ready.” It is more common for a writer to put something that is left out of a quote but should be there in brackets ([....]) rather than parentheses.

As @janbb stated, you would normally see square brackets used for this, but it might be a case of organizing the interview so that it makes more sense for the reader. For example, this:

Interviewer: Are you ready?
Nadal: Hopefully. I will try my best.

would become, in print:

“Hopefully [I am ready]. I will try my best.”

As for what it is called, it could fall under the rubric of paraphrasing.

I would agree with @janbb and @dappled_leaves .
It looks like an editorial addition to add context to his response that isn’t present in the article.

Since you don’t read/see/hear the interviewer asking him if he’s ready, you would have to include ”[I am ready]” to make sense of it.

I am ready (to play). It was an interjection.