Atletico, who have a game in hand, could have cut Palmeiras' lead to two points in Wednesday's late match but Marcos made a brilliant save to deny Renan Oliveira from the penalty spot on the hour after Wendel had brought down Thiago Feltri.
"It wasn't a penalty but happily I managed to save it," said Marcos, who played his 450th match for Palmeiras.
Striker Eder Luis put Atletico ahead at the Mineirao in Belo Horizonte after five minutes with a shot from outside the box that went in under Marcos, Brazil's 2002 World Cup-winning keeper.
Palmeiras equalised 10 minutes before the interval when Paraguayan striker Jose Ortigoza headed home Diego Souza's cross.
Souza and Cleiton Xavier missed good chances to win the match for coach Muricy Ramalho's team, who have 36 points from 18 matches, four more than Atletico and third-placed Goias.
Internacional, who have two games in hand, climbed over title holders Sao Paulo into fourth on Monday after a 3-0 win over bottom side Sport Recife.
Andres D'Alessandro, playing while Inter appeal his 60-day suspension for violent conduct during the Copa Brasil final against Corinthians last month, scored the third goal after coming on in the second half. The appeal will be heard on Friday.
