Stunning upsets have sent shockwaves across the footballing planet, and we are less than one week into the 2018 World Cup.

Brazil, Germany, Argentina and Spain all failed to win their respective openers - the first time this has happened at a World Cup.

Spain, still reeling from the sacking of their coach Julen Lopetegui, had no answer to Portugal's one-man machine Cristiano Ronaldo in a thrilling 3-3 draw. Then Lionel Messi's Argentina froze against Iceland in a 1-1 stalemate.

It was Sickening Sunday for Germany and Brazil as Joachim Low's men suffered a "Chucky" nightmare in the form of Hirving Lozano in a 1-0 loss before Tite's Selecao were held 1-1 by the stubborn Swiss.

France scraped past Australia 2-1 as they were helped by VAR and a deflected shot.

So what has gone wrong for the traditional powerhouses?

THE FEARLESS FACTOR

Debutants Iceland best exemplify this fearless attitude. Whatever they lacked in skill, they compensated with their height, strength and tenacity.

The Vikings covered every blade of grass and there always seemed to be a head, leg or other Icelandic body part in the way of the Argentina players. Nine of Iceland's starting XI were 1.83m or taller. Nine of Argentina's starters were under 1.83m. Iceland used every spare inch to their advantage.

Argentinian legend Diego Maradona criticised coach Jorge Sampaoli, saying: "It's a disgrace. Not having prepared for the match knowing that Iceland are all 1.90m tall... I can't blame the players, much less Messi, who gave it all he had."

The same can be said of European champions Portugal (world No. 4), Switzerland (6) and Mexico (15). All were no pushovers and were expected to progress from the group stage.

Instead of sitting back, they took the game to their more illustrious opponents and their brave approach worked wonders.

FIGURING OUT A GAME PLAN

It was clear the underdogs did their homework and studied their opponents thoroughly.

Mexico stunned Germany with their pace, which coach Juan Carlos Osorio said: "We started drawing up a plan six months ago, we had to change some parts because of injuries, but basically using players who are very fast down the flanks."

Nippy forward Javier Hernandez started up front despite a disappointing season for West Ham, and it paid dividends as it was his quick release that allowed another speedster Lozano to cut in and score.

In the case of Iceland's historic draw, goalkeeper Hannes Halldorsson revealed that he had studied Messi's penalties before going the right way to thwart the five-time Ballon d'Or winner.

TACKLING THE TALISMANS

Switzerland put the brakes on Neymar by committing 10 fouls on the Brazilian - the most on a single player at the World Cup since Tunisia manhandled Alan Shearer 11 times in 1998.

Messi found the going tough against Iceland's two banks of four, and never quite managed to break into the box like he usually does.

Sticking to Messi like leeches, two or three Icelandic players kept a tight rein on him. On the rare occasion that he managed to find a hole in the defence, others stepped in to plug the gap.

Germany's key attacker Thomas Muller also found little space or joy on the right of a 4-2-3-1 against a resolute Mexican defence.

DISSECTING DODGY DEFENCES

Brazil, Germany and Spain had the best defensive records during qualifying. But the underdogs found the chinks in their armour.

Mexico coach Osorio identified Germany's weak links in full-backs Marvin Plattenhardt, and especially Joshua Kimmich. Both were often caught too high up.

Germany centre-back Mats Hummels said: "If seven or eight players are only attacking, we lose the defensive stability. Jerome (Boateng) and I were often alone at the back. We were mercilessly countered."

Iceland midfielder Gylfi Sigurdsson told The Straits Times in an earlier interview they could capitalise on his familiarity with error-prone English Premier League defenders like Marcos Rojo and Nicolas Otamendi, and it was his shot that created panic among the duo before Alfred Finnbogason equalised.

Ronaldo also took advantage of the mismatch against Real Madrid team-mate Nacho when he won the early penalty from which he scored.

All is not lost for the favourites though, as there are still two group games for them to make amends.

Argentina lost their 1990 opener and still made the final, which they lost to Germany, while Spain went one further when they won the 2010 tournament despite starting on the wrong foot with a 1-0 defeat by Switzerland.