Club America set up Club World Cup semi against Real Madrid
Kim Bo-Kyung swept the South Koreans in front after 23 minutes in Osaka before the Argentine forward pounced twice to turn the tide in favour of the CONCACAF title holders.
Romero equalised with a clinical header just before the hour mark and volleyed home the winner via a slight deflection off Jeonbuk forward Kim Shin-Wook after 74 minutes as the K-League side wilted under increasing pressure.
America, who also beat Jeonbuk in the quarter-finals of the FIFA tournament 10 years ago, will face European champions Real Madrid in Yokohama on Thursday.
The hulking Kim Shin-Wook, instantly recognisable with his peppermint green mohican, was a constant menace butÂ was well shackled in a tense finish as the Mexicans clung on for victory.
South Africa's Mamelodi Sundowns were playing Japanese champions Kashima Antlers later on Sunday for the right to face South American Libertadores Cup holders Atletico Nacional in the competition's first semi-final on Wednesday.