The last team to qualify for the tournament, Uruguay needed to beat Costa Rica in a playoff to get to South Africa. The Uruguayans have looked like anything but an outsider, though, and on Tuesday beat Mexico 1-0 to win Group A.
"We knew we had two results that benefited us," said Luis Suarez, who scored the only goal. "But before the match our goal was to get the three points and be group leaders.
"Luckily we reached out first goal, which was to advance from the first round."
The Mexicans still advanced to the knockout round at the expense of host South Africa. Uruguay, which did not yield a goal in the first three matches, and Mexico will face the survivors from Group B, to be decided Tuesday night.
South Africa and France did not advance after the hosts beat the French 2-1. Mexico beat out the hosts on goal differential (plus-1 to minus-2).
Uruguay ended the group phase with seven points after overpowering South Africa 3-0 and drawing 0-0 with France.
South American teams have won nine of their 11 matches thus far.
Mexico earned four points, but a better goal differential than South Africa (plus-1 to minus-2). It has advanced in every World Cup it qualified for since failing to get out of the first round in 1978.
Suarez scored the winner in the 43rd minute after running unmarked to the back post and heading in Edinson Cavani's brilliant cross for his first goal of the tournament.
The two countries went into the last group game sharing the lead at four points and needed only a draw to advance. But both fielded attacking lineups. Even so, the game yielded few clear scoring chances.
Uruguay coach Oscar Tabarez said he was "bothered" by talk before the game that the two teams might play for a draw.
"It's an insult to the coaches and players. Both teams came out looking for something," Tabarez said.
Three quick thoughts on Mexico-Uruguay
Uruguay is a two-time winner of the World Cup, but those came in 1930 and 1950. It has not been a championship contender in recent tournaments and last advanced in 1990 in Italy.
This squad could be different. It has scoring power in Suarez and Diego Forlan, and its defense was unbeatable in the opening round.
"The important thing is that we qualified in first place," Forlan said. "They controlled the ball well and in truth it was quite hard for us."
Mexico coach Javier Aguirre preserved an attacking lineup that helped his team beat France 2-0 in its last game. Veteran striker Cuauhtemoc Blanco, at 37 the oldest field player at the tournament, replaced the injured Carlos Vela.
But the Mexicans never found the net. Its best chance came in the 22nd minute when Andres Guardado, in for suspended midfielder Efrain Juarez, fired a left-footed shot from 30 yards that bounced off the underside of the crossbar.
"It leaves a bitter taste," Mexican midfielder Rafael Marquez said of the outcome. "We're all unhappy with this result and with today's performance. We'll have to improve now."