After struggling in group play, the Americans dominated the Koreans, winners of Group D, getting two goals from forward Courtney Verloo, one from forward Vicki DiMartino -- the fourth straight game she has scored -- and a thunderbolt strike from Kristie Mewis.
The Americans outshot South Korea, 20-5, and got strong performances from the center back Cloee Colohan, outside backs Crystal Dunn and Rachel Quon and defensive midfielder Amber Brooks.
"I was really proud of the effort," said U.S. coach Kazbek Tambi. "The kids got a bit better with every game and culminating with their best game today so it was great to be able to play the best game in the World Cup quarterfinals."
The key goal was Mewis' goal that gave the Americans a 2-0 lead early in the second half, shortly after the Koreans came close to being awarded a penalty that could have tied the score.
She crushed a left-footed shot from about 23 yards out that flew into the upper right corner, giving Korean goalkeeper Lee Hyo Ju no chance.
"It was a fantastic shot," Tambi said. "We told them that given the number shots from outside that have gotten in during in this tournament, that we had to get that into our game as well and Kristie hit a beauty. There was no better goal on the day. I'm as much of a fan of the game as a coach and that was a great goal."
Germany eliminated Canada, 3-1, on Saturday. In the other semifinal, England will meet North Korea. England upset Japan, which had beaten the USA and scored 17 goals in three group games, on penalty kicks after their game ended in a 2-2 tie. North Korea dominated Denmark, 4-0.
Germany was the only group winner to survive the quarterfinals.
Nov. 9 in Hamilton
USA 4 South Korea 2. Goals: Verloo 27, 78, K.Mewis 54, DiMartino 84; Lee Hyun Young 65, 85.
USA --Vancil, Dunn, Colohan, Quon, S.Mewis, Brooks, Roberts, K.Mewis, Klei (Tymrak, 81), DiMartino, Verloo (Johnson, 86)
South Korea --Lee Hyo Ju, Seo, Lee Eun Kyung, Song (Lee Mi Na, 82), Shin, Oh Yu Sun, Koh, Oh Hye Jin (Park, 38), Lee Hyun Young, Ji, Lee Min Sun (Jeoun, 74).
Referee: Kirsi Savolainen (Finland).