Reigning champions Barcelona are third in the table and trail Carlo Ancelotti's men by four points with 10 league matches remaining.
Barca face a daunting task at the Bernabeu, where Real have racked up nine league victories in a row.
Indeed, the last time the league leaders tasted defeat in Spain's top flight was in the reverse fixture in October.
In contrast to Madrid's fine form, Barcelona have stuttered lately and had lost three out of six league matches before Sunday's 7-0 demolition of Osasuna at Camp Nou.
Midfielder Iniesta is confident of success, however, and pointed to the Osasuna rout as proof the side are capable of achieving the win.
Speaking to Spanish radio station SER, he said: "The truth is that if we want to maximise our options of retaining the league title, we have to win at the Bernabeu.
"Madrid are the leaders and have some of the best players, but form doesn't always count in these games.
"It's always more positive to go into a game like Sunday's after a good result - it helps you believe we can go there and win."
Tensions between the two sides often spill over in the hotly contested fixture, but Iniesta believes both sets of players will keep themselves in check on Sunday.
"They are always a difficult team to beat," he added. "But I don't think that we'll see the same tension of a few years back on the pitch. It'll be a war, but a footballing war and we mustn't step over that line."
