BELO HORIZONTE, Brazil (AP) - Shortly after coming on as a second-half sub in Argentina's last World Cup warmup, Lionel Messi doubled over and appeared to vomit on the pitch.
He's done it at least a half-dozen times with Argentina and club team Barcelona, mystifying doctors and fans alike.
"Nerves," said Argentina coach Alejandro Sabella, and that's a diagnosis as good as any.
The Argentina captain and four-time world player of the year is under pressure to lead the Albiceleste to its first World Cup trophy since 1986.
"I reckon that in these moments there is anxiety more than anything," Sabella said before the team departed for Brazil. "It's difficult to remain calm."
Top athletes throwing up before, during or after a competition is not unusual. Sometimes it's because of the physical stress on their bodies, sometimes because of performance anxiety.
What's unusual about Messi is it seems to happen quite randomly, not when the pressure would seem the greatest or when he's exhausted himself to the limit.
After Messi threw up less than 10 minutes into a friendly match between Argentina and Romania in March, his coach at Barcelona, Gerardo Martino, said "something is not right," though he added it wasn't affecting Messi's play.
Messi made that clear Saturday against Slovenia, scoring Argentina's second goal just four minutes after TV cameras showed him dry-heaving and receiving a tablet from the bench.
Barcelona's medical staff hasn't been able to find the cause. Neither have Argentina's team doctors, nor Messi himself. The decorated forward tends to not make a big deal of it, saying it's just something that happens to him in training, during matches and even at home.
"I don't know what it is. But I had a thousand exams," Messi told Argentine broadcaster TyC Sports earlier this year.
The pressure on him is huge. Even though he's just 26, his impressive record with Barcelona, including six Spanish league and three Champions league titles, has already brought comparisons with the all-time greats. His detractors like to point out, though, Messi has not won a World Cup. He's only scored one goal and never gotten past the quarterfinals in his two attempts on football's biggest stage.