MIAMI - Mexico head into Wednesday's World Cup qualifier against the United States knowing that failure to win could leave their hopes of reaching next year's finals in the balance. But after their 5-0 win over the U.S in the Gold Cup final, midfielder Andres Guardado believes there is a new mood of confidence which should serve them well at the Azteca Stadium against Bob Bradley's team. "After the good result we got in the Gold Cup the atmosphere we breathe within the group is different, lots of optimism and joy, and that helps for such important kinds of matches," Guardado said. The U.S. fielded a second string team in the Gold Cup but the convincing nature of Mexico's performance indicates they have turned the corner after a dreadful start to qualification. After defeats to the U.S., Honduras and El Salvador - Mexico replaced coach Sven Goran Eriksson with Javier Aguirre but are still in fourth place in the six-team qualification group. In the CONCACAF region, covering North and Central American and the Caribbean, the top three teams qualify automatically for the finals in South Africa while the fourth placed team must playoff against the fifth-placed team from South America. Costa Rica lead with 12 points with the United States on 10. Mexico trail third-placed Honduras by a point but fifth-placed El Salvador are only a point behind Aguirre's team. "We are in a delicate situation where if the result goes against us our ticket (to the World Cup) gets very complicated, we must take advantage of being at home," said defender Radael Marquez, who is out of Wednesday's game at the Azteca stadium through injury. It would be a major upset if Mexico, the traditional powerhouse of the region, were to fail to make it to South Africa but with half the qualifying stage remaining automatic qualification is by no means out of reach. "We have been through various changes, we lacked balance. Now with Javier (Aguirre) and the Gold Cup result, we are more relaxed, we've improved a lot and are confident of what we can achieve now," said Marquez. Carlos Vela and Giovani Dos Santos, who play in England for Arsenal and Tottenham, are among those from the Gold Cup team in Aguirre's squad for Wednesday's game. Aguirre has to make some tricky choices in his attack where he also has Cuauhtemoc Blanco, Nery Castillo and Miguel Sabah at his disposal.