It was Croatia's heaviest ever defeat and the second time Spain have scored six times this year, having thrashed Argentina 6-1 back in March.
Star man Luka Modric will come up against many of his club team-mates for the visitors, and global team-mate - and Barcelona player Ivan Rakitic - is happy to have the Real Madrid man on his side.
Spain's Sergio Ramos celebrates scoring their fifth goal with teammates.
Croatia's biggest defeat had been a 5-1 away loss against England at Wembley in 2009. "We've played two teams that went very far in the World Cup and these victories are very important to reach the final phase, which is a challenge we have set ourselves", added Asensio. Spain was coached by Fernando Hierro in Russian Federation after Julen Lopetegui was sacked two days before the tournament. They replaced Marcos Alonso, Thiago Alcantara and Iago Aspas.
Tuesday's match in Elche marked ex-Barcelona boss Luis Enrique's first home game as Spain manager, and his second win in as many matches since taking over La Roja reins in July.
The hosts will need to be in better form against Romelu Lukaku and Eden Hazard than in their last game, the first without influential manager Heimir Hallgromsson, where they were beaten 6-0 by Switzerland.
Belgium had defeated Scotland 4-0 in a friendly last week in Glasgow.