Neymar and Co. were jeered off the pitch by the fans while Argentina crashed to their first defeat in the Olympics since 1996. South Korea went berserk in the second half to trounce Fiji 8-0. Iraq dominated Denmark but failed to find openings in the game, playing out the second 0-0 draw of the tournament.
Mexico and Germany were in an entertaining 2-2 draw, with the world champions barely getting their equaliser.
Nigeria and Japan's encounter was edge-of-the-seat stuff with the teams scoring nine goals between them. Honduras too, posted a win.
Brazil 0-0 South Africa
In Group A, Neymar's men were held to a goalless draw by South Africa, who, despite having to play with a man down in the last half hour of the game, were staunch in defence. The best chance of the match fell to Manchester City's latest signing, Gabriel Jesus, who smashed the ball against the post despite an empty net. Neymar was left with too much to do on his own.
Sweden 2-2 Colombia
In Group B, Sweden and Colombia faced off at Manaus, serving up a thrilling draw. Sweden made a bright start to the game, but it was the South American side that went into the lead. Dorlan Pabon put skipper Teofilo Gutierrez in sight of goal, and the finish was calm and collected.
Mikael Ishak looked threatening for Sweden, levelling scores with an excellent shot from outside the box. On the hour mark, the Scandinavians took the lead with Astrit Ajdarevic pouncing on a rebound. Pabon equalised from the penalty spot for Colombia and the game swung back and forth till full-time.
Nigeria 5-4 Japan
With something like a tennis match scoreline Nigeria continued to feature in memorable matches at the Olympics. Nigeria's Etobo scored four to take his country past Japan in a goal-fest in a Group B game. The first goal of the game came from Nigeria's Sadiq Umar, who had the presence to get on the end of a rebound.
Shinzo Koroki equalised through a penalty. Etebo restored the lead for Nigeria, showing excellent ball-control and finishing prowess. Japan struck back in no time with Takumi Minamino making a stylish finish. The first-half ended 3-2 in Nigeria's favour as Etobo found his second.
Early in the second half, the Super Eagles got a penalty, and later, yet another Japanese defensive lapse handed the Nigerians a 5-2 advantage, Etobo getting his third and fourth goals.
Takuma Asano's delicate backheel narrowed the lead, and the excitement grew when Musashi Suzuku thumped a shot in from inside the penalty box. But Nigeria held on.
Mexico 2-2 Germany
Holders Mexico lived up to their pre-tournament billing and probably left the pitch kicking themselves for squandering the lead twice in the Group C game. German skipper Leon Goretzka limped off in the first half. It was not until the second half that the game opened up. Oribe Peralta headed the ball in for Mexico from a corner. Germany found an answer a minute later as Serge Gnabry sneaked into the area to wallop the ball home.
The highly-rated Rodolfo Pizarro put the Mexicans back into the lead as he scored off a rebound. Timo Horn denied Mexico from point-blank range before the Mexicans were denied three points by Mattias Ginter from a corner.
South Korea 8-0 Fiji
In group C, South Korea ran riot against Fiji as the goals kept raining in at the Fonte Nova Arena. Seung Woo Ryu netted a hat-trick for the Asian giants, who had to wait till the 32nd minute of the game to score. There was no looking back from there. The wheels started to come off for Fiji in the second half, with Korea scoring three in as many minutes early in the first half. Ryu completed his hat-trick, the second of the day, as there were four more goals late in the contest.
Honduras 3-2 Algeria
In one of the surprise wins of the day, the Hondurans got the better of the African side by a single goal. Romell Quioto gave Honduras the lead after following up on a rebound. Marcelo Pereira ensured that the Central American team went into half-time with a commanding lead as he headed in the second, the ball hitting the bar before rolling into the net.
Algeria looked the better team in the second half and were rewarded late in the game as Mohammend Benkablia's scrappy effort cut the lead down to a solitary goal. The Algerian 'keeper, Farid Chaal dropped a relatively easy take and Anthony Lozano punished the error to put Honduras 3-1 up. Algeria pulled one back through Baghdad Bounedjah's exquisite control and finish, leading to a tense finish.
Portugal 2-0 Argentina
In an upset of sorts, Portugal ran out emphatic winners against heavyweights Argentina. In a strange case of role-reversal, it was the South Americans who were happy to sit back and play on the counter-attack. Johnathan Calleri hit the post with a chipped shot in the second half.
FC Porto striker Goncalo Paciencia gave Portugal the lead as his drive from the edge of the ball left Argentina no chance. Pite then had a touch of fortune as the ball rolled into the net as Argentina's goalkeeper Geronimo Rulli made a mess of a straightforward take.