By Peter Rutherford
SAO PAULO (Reuters) - Lionel Messi has been the key to Argentina's perfect start at the World Cup, scoring four of their six goals in three straight group wins, and the South Americans hope his blistering form continues when they face Switzerland in the last 16 on Tuesday.
Also battling for a place in the quarter-finals are the United States and Belgium, who arrived as one of the favorites to lift the trophy but have not yet fired on all cylinders.
Although Argentina have yet to reach the heights many expected of them in Brazil, Messi has been on a different planet, electrifying crowds and banishing memories of the goal drought he endured in South Africa four years ago.
Fellow forward Sergio Aguero is a major doubt after picking up an injury against Nigeria in their final group game, however, and should he miss out manager Alejandro Sabella may opt for the versatile Ezequiel Lavezzi.
Sabella, though, was playing his cards close to his chest on the eve of the match in Sao Paulo.
"In case Lavezzi plays he can play in two different positions," he told reporters. "But I am telling you, the line-up is not decided yet."
Switzerland hope Xherdan Shaqiri, dubbed the 'Alpine Messi' for his impish skills, will provide the key to beating Argentina for the first time.
The bustling little forward has been in top form for the Swiss, stealing the show with a hat-trick in a 3-0 win over Honduras that sealed their spot in the last 16.
"The midfield will feed him, and he plays really well up front," captain Gokhan Inler said of Shaqiri. "The good thing is he cannot be figured out that easily. We need to have him free."
In Salvador, the United States will look to sprint out of the blocks to unsettle slow-starting Belgium.
The Belgians ground out group wins over Algeria, Russia and South Korea but were far from impressive, failing to score in any of those games until the last 20 minutes.
Defender Thomas Vermaelen has not recovered from a hamstring injury and will miss the game while captain Vincent Kompany will undergo a late fitness test.
But coach Marc Wilmots expects playmaker Eden Hazard, another one who has yet to hit full stride, to have a big influence on the game.
"I’m sure that we’ll see a massive Hazard performance tomorrow which could prove decisive," he added.
After muscling out Portugal and Ghana for second spot behind Germany in Group G, the United States have grown in confidence and plan to go on the offensive.
"We have no fear at all," U.S. coach Juergen Klinsmann said on Monday. "We feel fresh, we have recovered well after every game so far. We feel we have just started this adventure."
Klinsmann will be able to call on forward Jozy Altidore, who has recovered from a hamstring injury picked up in their group opener.
(Editing by Ed Osmond)