While the visitors have booked their spot in the next round, Olympiacos are third in the group and are in danger of exiting Europe with Marseille close behind.
Attempting to usurp one of Porto or Manchester City to a top-two finish in Group C was always going to be a step too far for Olympiacos, who have only managed to pick up one win from their first five in this year's tournament.
The Greek powerhouses travelled to a Marseille side who had not even chalked up a goal, yet alone a point, in Group C last week, but Olympiacos became Les Olympiens' first victims of the group stage as they lost 2-1 at the Orange Velodrome on matchday five.
A Mady Camara goal broke the deadlock just after the half-hour mark, but two second-half penalties from Dimitri Payet turned the tie on its head and ensured that Marseille would not be waving goodbye to the Champions League without something to show for it.
Pedro Martins's men still sit above Marseille in third prior to the final round of group fixtures, and while Olympiacos are only ahead courtesy of head-to-head away goals, they can take solace from the fact that Marseille will have to somehow avoid defeat at Man City to pip Olympiacos to third place.
The Greek Super League champions continued their dominance in the league with a 4-1 victory over Volos NPS at the weekend, with Youssef El Arabi netting a second-half hat-trick in that fixture.
Porto deserve immense credit for keeping Man City at bay last week, but the Portuguese side's strong defensive showing was overshadowed by a newsletter released after the game which took aim at several of Pep Guardiola's players.
Off-the-pitch matters aside, Sergio Conceicao's men are set for another go in the Champions League knockout stages after securing 10 points from five matches in Group C, with City the only side that Porto have failed to beat so far.
The Portuguese champions easily overcame Marseille twice without shipping a goal, and this match is little more than a dead rubber for Porto, who have won six of their last seven in all competitions.
Conceicao's men were not given an easy ride against Tondela at the weekend and only just scraped a 4-3 victory against the mid-table side at home, with Moussa Marega netting twice as Porto finished the game with 10 men.
Olympiacos Champions League form: WLLLL
Olympiacos form (all competitions): WWLWLW
Porto Champions League form: LWWWD
Porto form (all competitions): WWWWDW
Olympiacos right-back Mohamed Drager lasted all of five minutes before having to be replaced on Saturday, and the 24-year-old could reportedly be set for several weeks on the treatment table.
Olympiacos possible starting lineup:
Sa; Rafinha, Semedo, Cisse, Holebas; M'Vila, Camara, Bouchalakis; Fortounis, El Arabi, Masouras
Porto possible starting lineup:
Marchesin; Manafa, Mbemba, Leite, Sanusi; Grujic, Oliveira, Vieira; Corona, Taremi, Nakajima
We say: Olympiacos 0-1 Porto
It would be a surprise to see Porto name a full-strength lineup for this tie with their last-16 qualification already secured, and Olympiacos can afford to feel optimistic about their chances of overcoming the Portuguese giants this week. However, we still think that Conceicao's solid defensive contingent will produce the goods and lead Porto to a narrow victory on Wednesday.