Manuel Pellegrini's men destroyed the Hammers 6-0 at the Etihad Stadium earlier this month but the Argentine insists a place at Wembley is not yet assured
City romped to a 6-0 victory in the first leg at the Etihad Stadium two weeks ago to all but seal their final spot, where they will face either Sunderland or Manchester United.
The huge advantage will likely see boss Manuel Pellegrini rotate his squad at Upton Park, but Zabaleta is confident that those players who come into the XI will be determined to make just as big an impact.
"It doesn't matter about the first leg," he said.
"We have to go into that game thinking it's a new game, even though we have six goals.
"We need to try and win it, like we are doing in other games at the moment. We are winning games and working hard.
"West Ham are in a tough moment, they are at home and will want to win it. So we need to take it seriously and we will."