Los Blancos have seen their 10-point advantage over title rivals Barcelona whittled down to just four points after a run of three draws in their past five games.
However, Karanka, sent out to face the media in place of coach Jose Mourinho on the eve of the Madrid derby, dismissed the suggestion that the leaders have lost their composure.
"We have no fear or anxiety," the assistant coach stated. "We are calm. There is no fatigue. The players are great and looking forward to [the game]. We must keep winning.
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Wednesday's game at the Vicente Calderon is also of the utmost performance to Atletico, who are seventh in the table, seven points behind fourth-placed Valencia.
Karanka is anticipating a fierce battle against a side who have improved dramatically since the appointment of Diego Simeone as their new boss during the winter break.
"Atletico changed their coach and what has improved the most is their character and unity," he mused.
"Atletico are a good team; they are in need of points and that makes this a difficult game."
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