One by one, they were all knocked out. Uruguay, Spain and Team GB.
Brazil are the only favourites left in the men's Olympic football tournament, so they have no excuse for leaving London 2012 with anything less than the gold.
"The pressure to win our first Olympics has always been there," Brazil coach Mano Menezes said. "We need to make it happen this time."
Brazil have brought most of their top players to the Olympics and has been dominant so far, getting more wins and scoring more goals than any other team. South Korea, meanwhile, just barely made it through the first stage, backed by a strong defence, but they are hoping for a second consecutive upset to add to their surprise elimination of host Team GB in the quarter-finals.
The winner of the match at Old Trafford will play for the gold against either Mexico or Japan, which will face off in London in the other semi-final.
"There is an extra bit of tension in the matches now," Menezes said. "You are playing to try to get to a final, it's different."
It would be only the third Olympic men's football final for Brazil, and the first since the 1988 Seoul Games. Brazil won the silver medal in Seoul and also four years earlier at the Los Angeles Games. Brazil got the bronze in 1996 in Atlanta and in 2008 in Beijing, when they lost the semi-final to Lionel Messi's Argentina.
"We know that our responsibility is increasing as we get closer to the final," Brazil captain Thiago Silva said. "And to get there we know that we will have a very difficult task trying to beat South Korea."
South Korea, who have conceded only two goals in four matches so far, are making their first semi-final appearance at the Olympic Games. The team failed to advance past the group stage in Beijing four years ago.
South Korea have won outright only one match in this year's tournament, a 2-1 result against Switzerland that helped it advance past the first round following draws with Mexico and Gabon. They drew 1-1 with Team GB in normal time and extra-time before eliminating the hosts 5-4 on penalties in the quarter-finals.
"We are very pleased to play against one of the top teams in the world in a big tournament like this," South Korea coach Hong Myung-bo said. "It will be very good for the players and their experience."
Brazil reached the semi-finals by eliminating Honduras in a difficult match on Saturday in Newcastle. Brazil struggled and twice had to come back from deficits against a Honduran team which played with 10 men from the 33rd minute because of a red card.
"We expect to play better than we did against Honduras," Silva said. "South Korea wouldn't have reached the semifinals if it didn't have a good team. They certainly can play."
The Mexicans arrived at the London Games as one of the medal contenders and were considered to have an outside chance for the gold. They are trying to improve from the fourth-place finish at the home games in Mexico City in 1968, when they lost the bronze to the Japanese.
Mexico were first in their group after victories against Gabon and Switzerland and a draw against South Korea. They eliminated Senegal 4-2 in extra time in the quarterfinals.
That third-place finish was Japan's best performance so far at the men's Olympic tournament. They reached the semi-finals by beating Egypt 3-0.
The Spaniards and fellow favourites Uruguay didn't even make it past the group stage, and when Team GB fell to South Korea, Brazil became the only top contender fighting for the gold.
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