Bale has featured in just seven of Madrid's 15 games this season due to an ongoing calf problem that saw him miss Wales' friendly with the Netherlands last week.
"I have been out due to injury for six weeks, so I haven't had the chance to play much this season," he told Madrid's website.
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"But I feel strong, in shape for this game and ready for the battle."
Bale's just over two-year spell in Madrid has been a largely disappointing one given the reported world record fee Real paid Tottenham Hotspur for his services.
However, his finest moment in a Madrid shirt came against Barcelona with a spectacular solo goal that handed Los Blancos a 2-1 victory in the Copa del Rey final in 2014.
An estimated television audience of 500 million is expected to tune in for what is widely regarded as the biggest game anywhere in the world.
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And Bale admitted the pressure of El Clasico is different to any other game in Spain.
"We are the two biggest teams in Spain. The rivalry is immense, it has been like that for many years and we will always compete for the league, the Champions League and many trophies.
"That is why there is such an incredible rivalry and all this adds pressure on the field, which is what makes the games so special and emotional."
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