Sydney have inflicted a costly defeat on Melbourne, with the Swans upsetting the AFL finals aspirants by 21 points in Cairns.
The Demons failed to consolidate their spot in the top-eight as they dropped their seventh game of the season as the Swans cruised to a 10.7 (67) to 6.10 (46) victory at Cazaly Stadium.
After a last-start confidence-boosting three-point victory over finals contenders St Kilda, Melbourne put together a dismal second quarter on Thursday to trail Sydney by 28 points at halftime.
The injury-hit Swans took full advantage of kicking with a strong breeze, whereas Melbourne didn't capitalise on the conditions in the first term.
Sydney did as they pleased through the middle, with co-captains Luke Parker and Josh Kennedy at their dominant best, as the Demons' pressure was near non-existent.
Swans youngster Justin McInerney broke through for his first career goal, and quickly added a second just before the main break to stun Simon Goodwin's team.
Melbourne lifted their intensity in the second half but the Swans' defensive game stood up and they only conceded one goal in the third term.
Parker's second major three minutes into the last quarter lifted the Swans back out to a 28-point lead before Melbourne began to look threatening.
Ed Langdon had a chance to cut the deficit to nine points with six minutes remaining but the Demons midfielder hooked his set-shot from 25 metres out.
Sydney immediately switched it down the other end of the ground as small forward Lewis Taylor iced the Swans' fifth win of 2020.
"It was always off the foundation of a really hard start to the game," Swans coach John Longmire said.
"We had 21 tackles in the first quarter against the wind; we wanted to be even with them at halftime and to kick a couple of goals against it (in the first quarter) was a bonus."
After starting the match in a golden position to move towards securing only their second finals appearance in 14 seasons, the Demons could find themselves outside the top eight by the end of the round.
"Incredibly frustrating first half. I thought our start was really poor," Goodwin told reporters.
"Sydney were really good, they came, they hunted.
"Our execution wasn't to AFL standard and we paid the price in that first half."
Swans defender Callum Mills was a late out due to a calf strain and was replaced by key forward Hayden McLean.
Veteran Demons midfielder Nathan Jones ended the match on the bench with a leg injury, while Angus Brayshaw also pulled up sore.
Longmire said Swans ruckman Callum Sinclair finished with a niggle and will need to be assessed.