Club America will face European champions Real Madrid in Yokohama on Thursday.
Local favourites Kashima Antlers salvaged some pride for Asia, however, when the Japanese title holders toppled South Africa's Mamelodi Sundowns 2-0 in the day's second quarter-final in Osaka.
In the early kickoff, Kim Bo-Kyung swept Korean powerhouse Jeonbuk in front after 23 minutes before Romero pounced twice to send the CONCACAF representatives through to the last four.
"The players showed character, attitude and heart," America coach Ricardo la Volpe said.
"We made some changes and tweaked the system a bit in the second half and got the goals.
"We passed a test today and we will play Real Madrid with no fear," he added. "We will have to work twice as hard but everyone expects us to lose so there will be no pressure on us."
Romero equalised with a clinical header just before the hour mark and the Argentine striker volleyed home the winner via a slight deflection off Jeonbuk forward Kim Shin-Wook after 74 minutes as the K-League side wilted under the mounting pressure.
Club America, who also beat Jeonbuk in the quarter-finals of the FIFA tournament 10 years ago, will face European champions Real Madrid, led by talisman Cristiano Ronaldo, in Yokohama on Thursday.
The hulking Kim Shin-Wook, instantly recognisable with his peppermint green mohican, was a constant menace but well shackled by markers in a tense finish as the Mexicans clung on for the win.
Kashima, who came from behind to beat part-timers Auckland City 2-1 in the tournament's curtain raiser on Thursday, were gifted their opening goal by Sundowns goalkeeper Denis Onyango in the 63rd minute.
Latching on to a knock-down from Shoma Doi, Yasushi Endo scuffed his left-foot shot which somehow squeezed under Onyango, bobbling out of his despairing grasp and over the line almost in slow motion.
Substitute Mu Kanazaki killed the game off by smashing home from close range with two minutes left after more sloppy defending from the Africans.
Kashima, fresh from clinching their eighth J-League crown last weekend, will face South American Libertadores Cup holders Atletico Nacional of Colombia in the competition's first semi-final on Wednesday.
"We struggled at the start to deal with their spirit," said Kashima coach Masatada Ishii.
"But we settled down in the second half, found a nice balance and scored two good goals. It was important to stay calm and we will have to be just as patient against Nacional."
Spanish giants Real are scheduled to arrive in Japan early Monday morning local time.