Brazilian side Gremio claimed a 1-0 home win over Colombia's Millonarios to give themselves the edge in their Copa Sudamericana quarter-final tie.
Any draw in the second leg on November 15 will give Vanderlei Luxemburgo's side a place in the penultimate stages of the South American club championship.
Marco Antonio scored the winning goal in the 36th minute, heading home a Ze Roberto corner which had been deflected into his path by Gilberto Silva.
Millonarios goalkeeper Luis Delgado had a strong second half, making several good saves to keep his team in the tie. He was beaten in the final minute by Andre Lima's header from a Ze Roberto free-kick - but the effort was ruled out for offside.
Midfielder Marquinhos said afterwards: "The main goal was a victory, so a 1-0 scoreline is good. Now we'll go to Colombia to try to score more."
The winners of this tie will meet Paraguayan giants Cerro Porteno or Argentina's Tigre.
The other quarter-final ties will see reigning champions Universidad de Chile clash with Brazilian giants Sao Paulo, while 2010 champions Independiente of Argentina meet Chile's Universidad Catolica.
Information from the Press Association was used in this report.