The Demons were sure of finishing fourth and facing Collingwood in a qualifying final before the first bounce but fine-tuned for September with an 11.11 (77) to 7.14 (56) win over the Swans on Sunday.
The Swans, needing a win to secure a sixth-placed finish and a home final, took a four-point lead into the last term but missed their chance as Fritsch sealed the result with three late goals.
The Demons (16-7) looked more settled near goal with Fritsch back in the side for the first time since breaking a foot in round 16, and the forward made an immediate impact with two early majors.
Fritsch looked sharp throughout and finished with five goals despite spending periods on the interchange bench receiving treatment on his foot before playing out the match.
Fellow medium forward Jake Melksham was a lively partner near goal and kicked two goals before being subbed out in the third term after suffering a knock on the knee.
Christian Petracca (29 disposals, one goal) helped spark the Demons in the second half, although he failed to make the most of several chances near goal, while Jack Viney (29) and Clayton Oliver (24) continued their strong form.
Key defenders Steven May and Jake Lever were also important as they controlled the backline even as the Swans tried to find avenues around them.
The Swans had the momentum in the third term and pulled away to a 17-point lead before Melbourne fought back with two late goals to stay in the contest before eventually overrunning the hosts.
Errol Gulden led the Sydney effort with 42 disposals and two stunning goals.
Co-captains Callum Mills (30, one) and Luke Parker (23) gave the Swans drive out of the middle along with Chad Warner (22), while retiring ruck Tom Hickey, playing his 150th match, engaged in a spirited running battle with Demons skipper Max Gawn.
The Swans (12-10-1) had their chances throughout but paid the price for misfiring near goal as Logan McDonald (1.3), Isaac Heeney and Parker (both 0.2) all missed gettable shots with game on the line.
Last year's runners-up Sydney will now be on the road for an elimination final, with their opponents decided through the Carlton-GWS match later on Sunday.
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