。
She's hooked on tennis all day long.
Literal
She [topic-は] all-day tennis [in-に] absorbed is.
~に夢中 ('absorbed in / crazy about / engrossed in') is built around the noun 夢中 (literally 'middle of a dream'), which describes total absorption to the point of forgetting one's surroundings. Pairing it with the formal copula である gives a measured, descriptive flavor — almost biographical, like a profile note. Tennis (テニス) is a popular school sport in Japan — many junior high and high schools have competitive 部活 (club teams), and 'all-day tennis' easily evokes the schedule of a serious club member.