The titleholders laboured to a 1-0 win over Croatia on Monday to book their ticket for the quarter-finals as Group C winners, and the Real Madrid midfielder believes they need a combination of hard work and luck if they are to successfully defend their European Championship crown.

"If we want to repeat that (success), it'll definitely be another difficult task. We'll have to work hard, we'll need a bit of luck. This is how it is here, and let's hope we can combine those factors again," Alonso said.

"In football now, all the teams are well-matched. We have achieved great things in the not-so-distant past, but we also struggled a lot, you cannot forget that.

"Clearly, the important thing was to advance as group winners. We knew it wasn't going to be easy because Croatia are a good team and had chances to go through as well."

Spain will meet the runners-up of Group D in the quarter-finals on Saturday.