In Argentina's last World Cup warm-up game, Lionel Messi appeared to be doubled over and vomiting shortly after his introduction to the game as a second half substitute. 

The reason?  "Nerves," says Argentina coach Alejandro Sabella, having seen his star man be sick over half a dozen times for both club and country. 

Of course, the Barcelona talisman is Argentina's captain and go to man.  He is a four-time world player of the year and having not won the World Cup since 1986, Messi is under huge pressure to bring the trophy home to his beloved nation.  

Sabella described how "I reckon that in these moments there is anxiety more than anything.  It's difficult to remain calm."

Vomiting from nerves, is of course, not a phenomenon completely unheard of before.  Zinedine Zidane famously vomited before taking his penalty against England at Euro 2004.

Sometimes athletes will be sick due to physical stress on their bodies, but also from the mental battles that are faced at top level sport.

The vomiting, however, is not the puzzling part of Messi's problem, what seems most bizarre is the erratic nature of his nerves.  

Gerardo Martino declared that, "something is not right," after  Messi threw up less than 10 minutes into a friendly match between Argentina and Romania in March, and it was clear that Messi's mental battles were becoming rather exhausting. 

Against Slovenia on Saturday, however, Messi was seen dry-heaving and receiving a tablet only minutes before scoring Argentina's second goal, further complicating the bizarre nature of the Argentine's problems.  

The vomiting is not anything new for Messi and it is still unclear whether his sickness has a big effect on his performance.  

As early as 2011, Messi was sick during the Spanish Super Cup Final against Real Madrid and scored shortly after being sick on the touch line. 

The much decorated superstar has been unable to get to the root of the problem despite consultations with both the Argentina and Barcelona doctors, yet remains calm about his anxieties.

Earlier this year Messi told Argentina braodcaster TYC, "I don't know what it is. But I had a thousand exams, I start to feel nauseous to the point where I almost vomit, and then it goes away."

It would be easy to see why Messi might be feeling anxious at a time when a large part of his country's World Cup hopes rest on his shoulders.

So often compared to footballing greats Pele and Maradona, Messi has so far under-performed at his two previous World Cups, scoring one goal and failing to make it past the quarter-finals stage. 

Messi must ensure that he remains focused this summer and plays with the ruthlessness and freedom that has been so devastating since he burst onto the scene as a shaggy haired kid nearly ten years ago. 

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