Netherlands rocked Spain in Salvador with a thumping comeback win, thrashing the World Cup holders 5-1.
La Roja went 1-0 up in the first-half after Diego Costa was awarded a contentious penalty; Xabi Alonso converted from the spot.
But if Spain were expecting a repeat of their 1-0 victory over the Dutch in the 2010 World Cup final, they were in for a shock.
Robin van Persie levelled just before the interval with a wonderful diving header, and Louis van Gaal's side went on to score four unanswered goals in the second half -- Arjen Robben (2), Stefan de Vrij and another from Van Persie -- as Spain were left bewildered.
In Group B's other encounter, Chile survived a spirited fightback from Australia to open up their World Cup campaign with a 3-1 win in Cuiaba.
Chile struck twice early in the match, and at that stage it looked like a rout was on the card. First on the scoresheet was Alexis Sanchez who latched onto a nod down from Eduardo Vargas to fire past Maty Ryan. And within two minutes Jorge Valdivia had doubled the advantage, collecting a pass from Sanchez to fire into the roof of the net.
But despite coming under pressure, Australia hit back 10 minutes before the interval when Tim Cahill headed home from an Ivan Franjic cross.
Australia pushed hard for a leveller and deserved to claim a point from the game, but it was not to be and Chile made sure of the win when Jean Beausejour arrowed a loose ball into the bottom corner in stoppage time.
Earlier, Mexico got their World Cup campaign off to a perfect start with a 1-0 win over Cameroon.
The winner came just after the hour through Oribe Peralta, Mexico's top-scorer in qualifying, who pounced on the loose ball after Charles Itandje had saved well from Giovani dos Santos to slot into the empty net, putting Brazil level on three points with hosts Brazil in Group A.
World Cup Watch: Day 2 wrap
ESPN FC's Steve Nicol, Shaka Hislop and Jay DeMerit discuss the Netherlands' win over Spain, Australia's effort against Chile, and Peralta's winner against Cameroon.