Defending champion Argentina matched the USA on Tuesday, scoring six goals in a 6-0 victory over Panama at the Under-20 World Cup.
Maximiliano Moralez and Sergio Aguero scored two each for Argentina.
Brazil, the 2007 South American champion, recovered from a stunning 1-0 loss to Poland in its first game to beat South Korea, 3-2.
The Brazilians raced ahead, 3-0, after an hour on a pair of goals by Alexandre Pato and a solo goal by Amaral, but the Koreans got goals in the last seven minutes from Shim Young Sung and Shin Young Ruk to make the game interesting.
UNDER-20 WORLD CUP RESULTS
Brazil 3 (Amaral 35, Alexandre Pato 48, 59) South Korea 2 (Shim Young Sung 83, Shin Young Ruk 89).
USA 6 (Szetela 9, 51, Adu 20, 45, 85, Altidore 70) Poland 1 (Janczyk 5).
Argentina 6 (Moralez 20, 27, Zarate 23, Agiero 25, 62, Di Maria 76) Panama 0.
Czech Republic 2 (Kalouda 56, Fenin 66) North Korea 2 (Kim Kum il 12, Jon Kwang Ik 89).