Teenage striker Joel Campbell inspired Costa Rica to a 2-0 victory over nine-man Bolivia to leave Argentina's Copa America future under threat.
A second-string Costa Rica side, missing almost half the squad that played at last month's Gold Cup, moved ahead of Argentina into second with three points, one ahead of Argentina and one behind leaders Colombia.
Tournament favourites Argentina were held 0-0 by Colombia on Wednesday, after dropping points in their opening fixture, a 1-1 draw with Bolivia.
Costa Rica, only at the tournament after they replaced Japan, who withdrew in the aftermath of the March earthquake and tsunami, play the hosts on Monday.
"In the second half I pulled Campbell over to the left wing where he was able to make a difference and his confidence grew," Costa Rica's Argentinian coach, Ricardo La Volpe told The Guardian. "After Argentina's result last night, this result today doesn't make things any easier for them."
Costa Rica striker Josue Martinez scored on the hour, after Bolivia goalkeeper Carlos Arias parried a shot from Allen Guevara. The 19-year-old Campbell scored the second in the 79th minute, hitting a crisp finish under the advancing Arias.
Bolivia had the defender Ronald Rivero sent off for blocking a goal-bound shot but Allen Guevara's penalty was well-saved by Arias.
But Bolivia were left with nine men in the 76th minute when Walter Flores was sent off for a foul on Campbell.
In the U.S., the tournament is being shown live exclusively on Spanish-language broadcaster Univision, with match highlights and live streaming also available on the company's website.
In the U.K., ESPN is broadcasting coverage of the tournament for the first time, a full list of live fixtures and highlights is available on the channel's website.
In addition, Google YouTube is streaming live matches to viewers in over 50 countries at youtube.com/copaamerica (not available in the U.K.). All matches are being screened live with English commentary including replays and highlights which are available on demand.