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Portugal have insisted they are not a one-man team and in Nani they have another dangerous forward, a fact acknowledged by Alonso, who netted both goals in Spain's 2-0 quarter-final win against France on Saturday.
"Portugal have forwards who can cause problems for us and above all the two wingers Cristiano and Nani are superb players," Alonso, who made his 100th international appearance against France, told reporters.
"We know Cristiano perfectly and he has been playing at an incredible level all season at Real Madrid," added the 30-year-old former Liverpool player.
"But the important thing is to focus on ourselves and make sure we are playing at our optimum level."
The Spanish players are also familiar with two of Portugal's defenders, centre back Pepe and left back Fabio Coentrao, who both play for Real.
"They also know how to close ranks in defence and they are very solid," Alonso said.
"Maybe before the game there will be a warmer greeting than usual because he's your club team mate but in the end you are going to be fighting your own corner," he added.
"Ultimately everyone is fighting for their own goals."
If Spain get past Portugal they will meet either Germany, whom they beat 1-0 in the 2008 European Championship final and who are already through to the last four, or the winners of Sunday's remaining quarter-final between England and Italy.
Vicente del Bosque's side are seeking to become the first nation to win back-to-back continental titles with a World Cup in between.