The two-time world champions have won all three matches in Brazil so far to qualify top of Group F, five points clear of Nigeria.
A 3-2 victory over Nigeria in their final group game saw Argentina show their potential for the first time in the competition, having stumbled to victories over Bosnia-Herzegovina and Iran.
Di Maria acknowledges that Alejandro Sabella's side have not been at their best, but believes Argentina are constantly improving.
"We go from strength to strength," he is quoted as saying by Marca.
"We have reached our best level yet.
"I still believe we can win the World Cup, we are more united than ever, we feel very strong."
Argentina face Switzerland in the last 16 on Tuesday and the Real Madrid forward is expecting an open, attacking game in Sao Paulo.
"They have good midfielders and forwards, but are not as solid in defence," he added.
"It will be a more open game, like the Nigeria match.
"We should be focused on not making any mistakes because if you lose at this point you're out."
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