After trailing by four points at the first break, Fremantle kicked five goals to two in the second quarter to take the advantage.
David Mundy has been excellent in the midfield, gathering 14 possessions.
Skipper Matthew Pavlich has also been important, with 14 possessions and two goals.
A Fremantle victory tonight would ensure a Subiaco Oval home final next week.
Fremantle 8.6 (54) leads Carlton 6.2 (38)