。
She's really into tennis right now.
Literal
She [topic-は] now tennis [in-に] is-absorbed.
熱中する ('be absorbed in, be passionate about, be hooked on') uses the kanji compound 熱 ('heat') + 中 ('inside') — being 'inside the heat' of an activity. It takes the focus of absorption with に. Synonyms with overlapping nuance include 夢中になる ('be lost in a dream'), はまる ('be caught in, hooked on'), and 没頭する ('plunge head into'). 熱中 is fairly neutral and works for hobbies, sports, studies, or even less wholesome obsessions. Tennis itself has a strong recreational following in Japan, with public courts and school clubs widespread.