Boca Juniors could only record a 1-0 home victory over Fluminense despite playing with an extra man for nearly an hour in the first leg of their Copa Libertadores quarter-final on Thursday.
Carlinhos saw red in the 34th minute when he picked up a second yellow card, but Pablo Mouche's goal in the 51st minute was all they had to show for their numerical superiority.
Carlinhos was given his marching orders shortly when he was shown a second yellow for stopping a Boca attack with his hand.
Despite that, in the last move of the first half Flu could have opened the scoring when Anderson's header rebounded off Facundo Roncaglia's right hand. However, Colombian referee Jose Buitrago did not give a penalty.
Six minutes after half-time, Mouche received the ball from Dario Cvitanich and made it 1-0 with a powerful angled shot from inside the box.
After that, Fluminense coach Abel Braga decided on damage limitation with three defensive changes, while Boca's only real threat came from Juan Roman Riquelme.
The best chance he created fell to Rolando Schiavi, but his effort was well saved by Diego Cavalieri.
"We always want to win at home whatever the margin. I don't think we were decisive enough in attack," said Riquelme. "Then were dedicated only to defending and that made it all the harder."
The second leg is next Wednesday in Rio de Janeiro.
In another match between Argentinian and Brazilian sides, Velez Sarsfield defeated Santos 1-0 in Buenos Aires.
Striker Mauro Obolo headed home Emiliano Papa's cross to give the hosts the lead after good work from Francisco Cerro and Augusto Fernandez.