Since the format was reintroduced three years ago, no team from Portugal's top tier has been able to secure their safety, and following last week's home loss, the Algarve club could now follow that trend.
Match preview
In a high-stakes meeting between the Primeira Liga's 16th-placed finishers and the side that came third in this term's Segunda Liga, Portimonense failed to make the most of home advantage in last Saturday's first leg.
The Alvinegros found themselves two goals down shortly after half time at Estadio Municipal de Portimao, and only a late own goal reduced their deficit ahead of this week's return.
Despite being boosted by recently beating local rivals Farense in the Algarve derby - recording their first win since early April - Paulo Sergio's side could not capitalise on having a huge share of possession and firing some 26 shots at goal.
After finishing 15th last year to scrape clear of danger, Portimonense were consigned to the promotion/relegation playoff this time around, having posted only eight league wins all season.
Now trailing in their two-legged contest for Primeira Liga status, the pressure will be on the Portimao club to stage a memorable fightback in Aves.
With free transport and tickets being made available to members and fans that undertake the long trip north, the Alvinegros should have some noisy backing this weekend - now, it is up to stars such as Helio Varela and Carlinhos to produce.
After UD Vilafranquense's demise last summer, Aves SAD - or AVS - were born into the Portuguese second division, and they have made an immediate impact.
The northern club finished third on their Segunda Liga debut, winning 20 times along the way, and after events in Portimao last week they are now within 90 minutes of promotion.
Jorge Costa's side had managed just one win in five matches ahead of the first leg, but goals from Anthony Correia and Bernardo Martins put them in the driving seat before returning to Estadio do CD das Aves on Sunday.
A well-travelled coach, and once a rugged defender for both Porto and the national team, Costa will bring plenty of experience into such a nail-biting occasion, as will his age-defying star forward.
Though he failed to find the net last week, former Cagliari striker Nene is still going strong, and the 40-year-old - who started his professional career some 20 years before his current club were founded - would love to make it back to the big time.
AVS form (all competitions):
W
W
L
L
L
W
Portimonense Primeira Liga form:
D
L
L
D
W
L
Team News
Despite being forced off by a collarbone injury during the first leg, Portimonense's Brazilian centre-back Alemao should be fit to face AVS on Sunday, after X-rays confirmed no fracture was sustained.
Filipe Relvas will be able to feature after serving a suspension last week, and he may also start in defence; however, Tamble Monteiro is still sidelined by a thigh problem and England-born Bulgaria international Sylvester is also unavailable.
In the latter pair's absence, tricky winger Helio Varela and captain Carlinhos - who has posted a career-high tally of 12 goals across all competitions this season - will be expected to support Ecuadorian striker Ronie Carrillo up front.
Meanwhile, AVS midfielder Fabio Pacheco was sent off for a second bookable offence at the end of the first leg so is suspended, and Jorge Costa will once more be without captain Luis Silva, who was recently handed a two-match ban for allegedly using racist language.
Veteran forward Nene is set to lead the line again, having netted 23 goals in 32 Segunda Liga appearances this term. The Brazilian will start just in front of last week's Man of the Match, Bernardo Martins - or 'Benny' - who scored his side's vital second goal in Portimao.
AVS possible starting lineup:
Simao; Teixeira, Correia, Clayton; Alaba, Lucca, Amorim, Akinsola; Benny; Mercado, Nene
Portimonense possible starting lineup:
Nakamura; Guga, Henrique, Alemao, Relvas; Fukui, Carlinhos, Ventura; Pereira, Carrillo, Varela
We say: AVS 1-1 Portimonense (AVS win 3-2 on aggregate)
With far less to lose and home advantage on their side, AVS can celebrate their first promotion to Portugal's top flight, just a year after forming from the ashes of Vilafranquense.
The hosts have scored regularly all season and are much more accustomed to winning games than their Primeira Liga counterparts.