Brazil brought most of its 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 its strong defence, but is hoping for a second consecutive upset to add to its surprise elimination of host Britain 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 semifinal.
"There is an extra bit of tension in the matches now," Brazil coach Mano Menezes said. "You are playing to try to get to a final, it's different."
It would be Brazil's only third Olympic men's football final, and 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 it lost the semifinal 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."
The South Koreans, which conceded only two goals in four matches so far, are making their first semifinal appearance at the Olympic Games. The team failed to advance past the group stage in Beijing four years ago.
South Korea has won 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. It drew 1-1 against Britain in regulation and extra time before eliminating the host nation 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 semifinals by eliminating Honduras in a very difficult match on Saturday in Newcastle. Brazil struggled and twice had to come from behind 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."
Mexico was first in Group A after victories against Gabon and Switzerland, and a draw against South Korea. It eliminated Senegal 4-2 in extra time in the quarter-finals.
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.
That third-place finish was Japan's best performance so far at the men's Olympic tournament. It reached the semifinals by beating Egypt 3-0. It won Group D after a draw against Honduras and victories over Morocco and gold-favourite Spain.
The Spaniards and fellow favourites Uruguay didn't even make it past the group stage, and when Britain fell to South Korea, Brazil became the only top contender fighting for the gold.
Brazil is playing with most of the players from its main senior squad because many of them make the tournament's under-23 age limit, including Neymar, Oscar and Alexandre Pato. Each nation can also use three players over that age at the Olympics.
"The pressure to win our first Olympics has always been there," Brazil coach Mano Menezes said. "We need to make it happen this time."