The 31-year-old was dropped for Los Blancos' recent 3-2 Liga defeat to Malaga by coach Jose Mourinho, with deputy Antonio Adan taking up residency between the posts instead.
However, speaking at a press conference after the side's first training session since the winter break, the Spain international stressed that he is doing all he can to work his way back into contention.
"I'm working hard in training to fight for my place [in the squad]," Casillas said.
"It is the objective that I am aiming for and it is the coach who decides whether I play in the next game or not. [Mourinho] has our full support and confidence.
"I am very well. I'm not a machine, I have to train every day and there are times when I don't feel good, but I feel fine.
"Ahead of each match, each player must prepare themselves in advance, both technically and in the eyes of Aitor [Karanka] and Silvino [Louro], the latter of whom I've learnt very much from.
"You have to fight and be available to the coach when he sees fit [to make the call]."
Madrid currently find themselves third in the Spanish top flight - 16 points behind leaders Barcelona - and Casillas has urged his colleagues to rectify the situation and use the turn of the year as a reference point for the revival of their title defence.
"No, the word [to describe our situation] is not tension," he added. "When things don't go well, it means we are low on morale.
"The players have to be wanting more and cannot be sad. Our goal is to remain ambitious. The day I have no ambition left is the day that I will not be sitting here.
"We are 16 points off the pace, it's not what we wanted but 2013 has arrived, and, as I said last week, what we have to do is try to cut the deficit.
"I know the situation is difficult but it's not impossible."