The World Cup and European champions suffered the 1-0 friendly defeat to the South Africans on Tuesday after putting in a poor performance in a narrow 2-1 win over Equatorial Guinea a few days earlier.
Alonso promised that Spain had the desire to go back-to-back at Brazil 2014 with the side primed to lift with competition points on the line.
"Yes, obviously (we played poorly), but in Brazil they will be competitive games," Alonso said.
"Sometimes these kind of friendly matches lack the required intensity, and as we've seen when you don't play with this kind of intensity, any team can beat you.
"We know what to expect, and I can assure you the motivation and will to win (the World Cup) will be there."
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