Nine straight victories will not be enough for Switzerland to secure the first place in the standings if they lose to Portugal, who are three points behind the group leaders, but boast superior goal difference.
The Euro 2016 champions are, therefore, just a win away from booking a ticket to Russia, where their opponents will finish first if they win or draw.
Portugal ended a long wait for a major international trophy by lifting the title at the Euro 2016 finals, and they have since continued the rich vein of form.
The Selecao did lose the opening fixture to Switzerland, but then went on to win next eight qualifiers, scoring a total of 30 goals.
It was Real Madrid superstar Cristiano Ronaldo who scored half of those, whereas in-form Andre Silva contributed another 8.
Portugal were far from impressive few days ago as they needed over an hour to break minnows Andorra, and it was the two forwards who again got on the scoresheet.
The Selecao now need to defeat Switzerland in order to finish top of their group and they are priced at 1.67 (4/6) to succeed in their quest.
The likes of Adrien Silva, Joao Cancelo, Pizzi, Nani and Raphael Guerreiro have failed to make the cut, but coach Fernando Santos has no fresh selection worries.
Probable starting line-up (4-4-2): Patricio – Cedric, Pepe, Fonte, Eliseu – Bernardo Silva, Moutinho, William Carvalho, Quaresma – Andre Silva, Ronaldo.
Switzerland were unable to show everything they’ve got at the Euro 2016 finals despite missing on a place in the quarter-finals after a penalty shoot-out.
Nati then showed a massive improvement and went on to win first nine 2018 World Cup qualifiers, albeit failing to secure the first place in the standings.
Switzerland need just one more point to cross the line, but avoiding defeat at Estadio da Luz will be no easy task in spite of the fact the team are after their 11th straight victory.
Roman Burki and Valon Behrami both had to withdraw through injury, whereas Tim Klose, Silvan Widmer, Gelson Fernandes and Valentin Stocker were overlooked.
Switzerland continued the bright run of results with a 5-2 hammering of Hungary on Saturday, but they will have bigger fish to fry on Tuesday.
Probable starting line-up (4-2-3-1): Sommer – Lichtsteiner, Schar, Djourou, Moubandje – Freuler, Xhaka – Shaqiri, F. Frei, Zuber – Seferovic.