South Korea staged a spirited comeback to deny the United States victory in a pulsating encounter in Daegu.
Clint Mathis fired the Americans into the lead midway through the first half with a sweetly-struck shot.
And Brad Friedel saved brilliantly to keep out Lee Eul-Yong from the penalty spot just before the break.
Friedel went on to make a series of stunning saves before South Korea scored a worthy equaliser courtesy of a glancing header by substitute Ahn Jung-Hwan with 12 minutes remaining.
It was no less than the competition's co-hosts deserved after dominating large passages of this compelling game and the result leaves both teams with a chance of progressing from Group D.
Given the highly-charged atmosphere that preceded the match - fuelled by the South Korean fans' growing expectations and US antipathy - the co-hosts started explosively.
A red sea of 60,000 adoring home supporters witnessed wave after wave of South Korean attacks.
Seol Ki-Hyeon and Hwang Sun-Hong were at the fore of South Korea's best moves, posing a constant threat to the US defence - and the former forced Friedel to make a good save.
Guus Hiddink's men totally dominated the opening exchanges, so it came as a massive surprise when the United States took the lead on 24 minutes.
A delightful chip from John O'Brien found Mathis, whose superb first touch was equalled by his left-foot strike that thundered past Lee Woon-Jae.
The goal buoyed the Americans, and Landon Donovan almost extended their lead soon after but was thwarted by the South Korean defence.
With half time looming, South Korea went in search of the equaliser and were awarded a penalty for their efforts when Jeff Agoos made a clumsy challenge on Hwang Sun-Hong.
Lee Eul-Yong took responsibility for the spot-kick, but Friedel saved brilliantly and Kim Nam-Il failed to put away the follow-up.
America were forced to withstand constant pressure in the second half and had the excellent Friedel to thank for keeping the South Koreans at bay.
Twice the giant keeper came to his side's rescue - first pulling off a fabulous one-handed save from Seol Ki-Hyeon and then keeping out substitute Choi Yong-Soo's shot.
But he could not do anything to stop Ahn Jung-Hwan's glancing header in the 78th minute, which sent the South Korean fans wild with delight.
In the ensuing celebrations, Ahn Jung-Hwan imitated a speed skater - a stark reminder to the watching world that the South Koreans have not forgotten the gold medal one of their skaters was stripped of in the US Winter Olympics.
Choi Yong-Soo then almost grabbed the winner in the dying seconds, but blazed his shot over the crossbar when unmarked in the area.
South Korea: Lee Woon-Jae, Choi Jin-Cheul, Kim Nam-il, Yoo
Sang-Chul (Choi Yong-Soo 69), Kim Tae-Young, Seol Ki-Hyeon, Lee Eul-Yong, Hwang
Sun-Hong (Ahn Jung-Hwan 56), Hong Myung-Bo (capt), Park Ji-Sung (Lee Chun-Soo 38), Song Chong-Gug.
Subs Not Used: Cha-Doo-Ri, Choi Eun-Sung, Choi Sung-Young, Choi Tae-Uk,
Hyun Young-Min, Kim Byung-Jim, Lee Min-Sung, Lee Yong-Pyo, Yoon Jong-Hwan.
Goal: Ahn Jung-Hwan (78).
USA: Brad Friedel, Frankie Hejduk, John O'Brien,
Claudio Reyna (capt), Clint Mathis (Josh Wolff 82), Jeff Agoos, DaMarcus Beasley (Eddie Lewis 74),
Brian McBride, Landon Donovan, Anthony Sanneh, Eddie Pope.
Subs Not Used: Gregg Berhalter, Steve Cherundolo, Cobi Jones, Kasey Keller, Carlos Llamosa, Pablo Mastroeni, Tony Meola, Joe-Max Moore, David Regis, Earnie Stewart.
Booked: Franki Hejduk, Jeff Agoos.
Goal: Clint Mathis 24.
Referee: Urs Meier (Swi).