。
Yumi is good at tennis.
Literal
Yumi [topic-は] tennis [object-を] do-[nominal-のが] skilled be-[polite-です].
The pattern [verb]-のが上手 ('is good at doing X') uses の to nominalize the verb so 上手 can predicate it. The が inside comes from 上手の が-marked skill — 'is skilled at [the doing of X].' Note both the を for the direct object and the が for the nominalized skill.