Paraguay, inspired by a Roque Santa Cruz hat-trick, thumped Colombia 5-0 while Argentina overpowered the United States 4-1 in a glut of goals at the Copa America on Thursday.
Favourites Argentina survived an early fright against inexperienced opponents, going behind to an Eddie Johnson penalty before two Hernan Crespo goals set them on the way to an impressive win.
Colombia striker Alvaro Dominguez missed a penalty two minutes before Paraguay opened the scoring in the first game of the Group C double bill in Maracaibo.
The result prompted Colombia coach Jorge Luis Pinto to apologise to his countrymen.
"I lament the score," he said. "I'm sorry for Colombia and for my country."
Argentina had Juan Roman Riquelme back in the side after going back on his decision to quit and Juan Sebastian Veron playing his first competitive international since 2003 but looked in trouble early on.
Johnson won and converted a ninth-minute penalty to give the United States, who are playing as Copa guests and brought an experimental team, a shock lead.
Crespo equalised two minutes later but Argentina were then frustrated by a packed U.S. defence for nearly an hour.
Crespo finally put Argentina ahead in the 64th minute, running onto to Lionel Messi's pass to place his shot beyond Kasey Keller.
Substitutes Pablo Aimar and Carlos Tevez added two more late in the game as their opponents were forced to open up.
PATIENT GAME
"We knew it was going
"But we played with patience. We knew we had 90 minutes and we were always sure we would win the game."
Colombia dominated the first half of the opening game but squandered an excellent chance in the 28th minute when Dominguez's penalty was saved by Justo Villar.
Two minutes later, Santa Cruz broke clear of the defence to put Paraguay ahead.
The Bayern Munich striker added another just 22 seconds after halftime and completed his treble in the 79th minute.
Substitute Salvador Cabanas compounded Colombia's misery by adding two more in the closing minutes.
"We missed a penalty and then they took advantage of the spaces we left and the mistakes we made in defence," said a bewildered Pinto. "We dominated the game."
Paraguay coach Gerardo Martino agreed the penalty was decisive.
"The key moment was the penalty, which Villar saved," he said. "The team was ruthless. We didn't have much of the ball but we got to the opponents' penalty area and scored."
Meanwhile, Brazil, upset 2-0 by Mexico on Thursday, had more bad news when striker Fred broke his foot during training and was ruled out of the rest of the tournament.
Coach Dunga, whose team must beat Chile on Sunday to stay alive in Group B, admitted he hardly slept after the defeat and had watched the match twice more in a bid to correct mistakes.
(Additional reporting by Luis Ampuero in Maracaibo)