Manly upset Cronulla Sharks 14-12 on Sunday Stadium at Remondis Stadium and slipped out of the top four.
The Sharks could finish sixth depending on the result of Sunday night's Canterbury and Warriors clash.
The Sharks are on 32 points, and Melbourne, on the same points, have a superior percentage.
If the Sharks finish fifth, they play team eight in the NRL finals series which starts next weekend.
Manly have beaten Cronulla twice this season and won the Steve RogersCup; the trophy named in honour of the late champion centre Steve Rogers.
Before a bumper crowd of 19,146 the Sharks had a real battle with the visiting Manly team.
Manly led 10-8 at halftime in a game where they were playing only for pride, as they are of the NRL finals race.
In the final round of the competition the Sharks entered the match on 32 points, fifth on percentages, with Melbourne, also on 32 points, but fourth on percentages.
Manly were also playing their last match under coach, loyal club man and former Sea Eagles premiership player, Geoff Toovey, who ends his association with the club which began in 1988.
Cronulla got in front in the second half through a try to Gerard Beale which put them in the lead 12-10.
But with the Sharks attacking hooker Michael Ennis threw a pass that was intercepted by Manly second rower Mark Horo who raced downfield before passing to flying winger Tom Trbojevic.
Manly led 14-12 after this try and the score remained unchanged to the end.
The Sharks had several tries overruled by the video referee.
Sharks forward Wade Graham said after the game they missed some opportunities.
He said the team will focus on next weekend's clash with the Dragons.
Leader Man of the Match: Cronulla prop Chris Heighington.
Manly 14 (Jesse Sene-Lefao, Tom Trbojevic tries, Kieran Foran 3 goals) defeated Cronulla 12 (Wade Graham, Gerard Beale tries, Michael Gordon 2 goals).
Crowd: 19,146.
Finals Draw: The eighth-placed St George Illawarra Dragons tackle the fifth-placed Canterbury Bulldogs on Saturday, September 12 at 5.50pm at ANZ Stadium.
The loser bows out of the competition and the winner plays the loser of the Friday, September 11, Sydney Roosters versus Melbourne Storm clash to be played at Allianz Stadium, the following weekend.
Cronulla finished sixth and will play defending premiers Souths, who finished seventh, in the elimination final on Sunday, September 13, at Allianz Stadium.
The winner of this game plays the loser of the Saturday, September 12, clash at Suncorp Stadium between Brisbane and North Queensland final the following weekend.
