The Atletico Madrid man watched on as the reigning champions fell to a surprising 5-1 defeat to Netherlands on Friday, and believes that shows anything can happen in the FIFA World Cup.
Marc Wilmots' squad are expected to make an impact in Brazil after impressing in qualifying, and possess a squad filled with quality all over the pitch.
However, Alderweireld knows they need to be at their best in order to ensure a winning start.
"I think it (Spain's defeat) says a lot of the level at the World Cup," he is quoted as saying in Het Nieuwsblad. "Everybody expected Spain to win, but the Dutch won with five (goals).
"We see that there are no favourites.
"We have to play very good, very focused and we have a lot of quality but we have to work very hard."
Belgium are making their first appearance at the World Cup since 2002, when they were knocked out in the round of 16 by eventual winners Brazil.
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