It would be no exaggeration to call her one of Japan's finest tennis players.
Literal
She [topic-は] Japan [in-で] [leading-屈指's] outstanding tennis player [is-である] [quotative-と] say [even if-ても] exaggeration [is-not-ではない].
Several formal patterns layered together. 屈指の ('one of the finest') is a formal-register superlative qualifier. ~であると言っても過言ではない ('it's no exaggeration to say that ~') is a stock formula for hedged-but-firm praise — common in essays, profiles, and journalistic appraisals.