Brazil, Argentina start afresh

Injury-hit Argentina resume their bid for a berth at the 2018 World Cup on Thursday as Brazil head into the latest round of South American qualifiers hoping to build on their Olympic gold medal triumph.

In the five months since the last batch of games in South America's marathon qualifying campaign, both Brazil and Argentina have undergone massive upheaval, with each side parting company with their coaches.

Brazil kissed goodbye to 1994 World Cup-winning captain Dunga after a dismal campaign in the Copa America Centenario which saw the five-time world champions knocked out in the group stage.

Argentina meanwhile split with Gerardo Martino following his team's Copa America final defeat to Chile in June, a loss which prompted a tearful Lionel Messi to announce his retirement from international football.

Messi was subsequently coaxed into a u-turn by new coach Edgardo Bauza, confirming in August that he would continue to be available.

But the Barcelona superstar is uncertain for Thursday's showdown with historical rivals Uruguay in Mendoza, with the 29-year-old struggling to overcome a groin injury.

Messi has travelled to Argentina to join the squad and was seen taking part in light drills in training on Tuesday.

However at least two other key players -- Manchester City's Sergio Aguero and Paris Saint-Germain's Javier Pastore -- are both definitely out.

Aguero suffered a calf injury in Sunday's 3-1 win over West Ham while Pastore has a torn right calf muscle.

Argentina, who face a trip to Venezuela next Tuesday after the Uruguay game, are currently third in the standings with 11 points after six of 18 games in the round-robin format.

Uruguay, who will have star striker Luis Suarez and defensive linchpin Diego Godin back in the same starting XI for the first time since the qualifiers kicked off last October, lead the standings on goal difference from Ecuador with 13 points.

While Bauza faces a difficult start to his reign against the in-form Uruguayans, Brazil have a potentially awkward trip to the thin air of Quito to face Ecuador.

Although the Brazilians have an impressive overall record against Ecuador -- winning 24 of 30 games since 1942 -- the Selecao have never won a World Cup qualifier in Quito, which is situated a dizzying 2,850 meters (9,350 feet) above sea level.

New Brazil coach Tite, appointed in June, has drawn heavily on Brazil's victorious Olympic squad for Thursday's game against Ecuador, which is followed by a challenging date with Colombia in Manaus next week.

Barcelona superstar Neymar, Gabriel Jesus, Gabriel "Gabigol" Barbosa and four other members of the Olympic squad are in the 23 for two games.

Former Corinthians boss Tite warned Brazil -- who are outside the automatic qualification positions after winning just two of their first six games -- they could not afford to assume they would turn their campaign around. Brazil has never failed to qualify for a World Cup.

"We're not in the qualification zone," Tite said. "Of course there's a risk, if we don't accept to see the risk, we're fleeing reality.

"I'm here because we haven't got the results. But there has been quality in the work that's been done."

KEY MATCHES

Ecuador v Brazil (Friday 3:00am BST) 

Argentina v Uruguay (Friday 5:00am BST)