Leonardo Mayer pulled Argentina level with Brazil 1-1 on Friday in their first-round Davis Cup tie, defeating Brazil's Thomaz Bellucci 6-4, 6-3, 1-6, 6-3 on clay in the Argentine capital.
Mayer was in control most of the way in a match that lasted 2 hours, 39 minutes.
Brazil took a 1-0 lead after the first singles when Joao Souza defeated Carlos Berlocq 6-4, 3-6, 5-7, 6-3, 6-2 in a match that finished in five hours.
Souza failed to convert two set points in the third set, but rallied to win the last two sets.
The edge could come in Saturday doubles with the Argentine pair Federico Delbonis and Diego Schwartzman slightly favored against Marcelo Melo and Bruno Soares.
In reverse singles on Sunday, Mayer is to face Souza and Berlocq plays Bellucci.
Neither of the countries, which are fierce rivals in football, has won the Davis Cup. Argentina has been runner-up four times: 1981, 2006, 2008 and 2011. Brazil has never reached a final.