。
She likes playing tennis.
Literal
She [topic-は] tennis [object-を] play [thing-の] [subject-が] liked is.
Same structural frame as 'good at playing X,' just with 好き ('like') in the predicate slot instead of 上手. Both 好き and 上手 are na-adjectives that pattern as stative predicates of inner state, taking their object with が. The のが nominalizer is what lets a verbal phrase (テニスをする) sit in the が-marked slot. Drop the verb and you get テニスが好きです ('She likes tennis [as an activity]'), which is the more usual way to express preference for an activity in casual Japanese.