Current form also suggests that Portugal will be difficult to beat.

Fernando Santos' team qualified for the Word Cup by losing just one of its European qualifiers. Ronaldo and AC Milan's Andre Silva formed a formidable attacking partnership with 24 goals between them. In addition, Santos has instilled great unity and self-confidence in his squad, qualities that have been ever-present since the glorious summer of 2016.

If Portugal has proven it can win without Ronaldo, Argentina continues to be heavily dependent on Messi. Just as the Albiceleste can thank their No 10 for taking them to the final of the 2014 tournament in Brazil, Argentina owes its place in Russia to the mercurial 30-year-old.

Needing to win its final qualifier last October to book a World Cup spot, the two-time world champion beat Ecuador 3-1 in Quito, inspired by a superb Messi hat-trick.

While Argentina's attack continues to be the envy of rivals, with Manchester City's Sergio Aguero and Juventus pair Paulo Dybala and Gonzalo Higuain providing able support for Messi, the midfield is short of truly world-class game changers, bar perhaps impish Sevilla playmaker Ever Banega.

Coach Jorge Sampaoli has not stopped experimenting with players and formations since taking charge last June, but he still seems unsure about his best starting lineup.

Doubts were exacerbated by a 6-1 friendly loss to Spain in March. A lack of match fitness is also a concern for several key players, with Ramiro Funes Mori, Marcos Rojo, Lucas Biglia and Aguero sidelined in recent weeks.