"Everybody says Spain are among the favourites, together with Brazil and Argentina. Portugal is seen as an outsider, but it's on the pitch that we will see. We're in a very good moment right now," Mendes told reporters on Saturday.
The Portuguese finished second in Group G after an ugly 0-0 draw with Brazil but in their second game they impressed with delightful play that resulted in a 7-0 thumping of North Korea.
They will meet Spain, who beat Chile 2-1 to top Group H, on Tuesday in Cape Town.
"If we are well on the day and play a good game, we have every condition to carry on in the tournament," Mendes added.
The midfielder has regained a place in the squad after a five-year absence, benefitting from Pepe's lengthy knee injury.
He started in Portugal's scoreless draw with the Ivory Coast and against North Korea and came off the bench in the 64th minute against Brazil.
Portugal coach Carlos Queiroz believes, however, that his team can hit even better form during the finals.
"The best Portugal is still to come. That's my conviction. The players are motivated and confident, and have shown a constant desire to keep improving," he said.
