Pompey are the new favourites to lift the trophy in May and will tackle West Brom in their semi-final, while Cardiff are up against Barnsley.
"We're favourites because we're the only Premier League team in there. But if you look at the cup all year, you realise you have to be at your best to get through.
"It's not a day out in the semi-final, it's a tough game and we've got to be at our best to win the game.
"We want to win the semi-final and we want to win the cup."
Redknapp knows West Brom will prove difficult opponents in the last four and has been impressed by the job Tony Mowbray has done at The Hawthorns.
He said: "West Brom have done fantastic this year. They are a good side and have played some good football.
"Tony Mowbray has done a great job with them so it will be a tough, tough game. They're a big club with great tradition.
"They're a good footballing team, they've got good players. Kevin Phillips is fantastic, Dean Kiely is a top goalkeeper. I had him here and he played a big part in keeping us up."