The winners of the nine European groups qualify directly for Russia while the best eight of the second-placed teams take part in two-leg playoffs for four more places.
Julen Lopetegui's Spain side scored three goals in the first 16 minutes to ensure the rest of the game was a formality against the overwhelmed hosts.
Spain beat Liechtenstein by the same score in Lopetegui's first match in charge last year, and this was another example of the verve and style he has his team playing with.
"I get on very well with (Morata) off the pitch too and that helps," Aspas told Movistar. "It's a pity that neither of us got a hat-trick, but the important thing was the three points.
"It was a great game in which we played without putting down our arms, scoring as many goals as possible in case we end up level on points with Italy at the end of the qualifiers.
"We knew the difficulty of the game, the coach warned us about France's game at home against Luxembourg (a 0-0 draw), which was an example for us."
QUICK START
The Spanish took control from the start with Sergio Ramos heading home David Silva’s free-kick from close range in the third minute.
Chelsea striker Morata netted the second in the 15th, nodding in Andres Iniesta's cross, before Liechtenstein goalkeeper Peter Jehle put the third goal on a plate for midfielder Isco with a horrific mis-kick.
David Silva added the fourth with a free-kick before the break and halftime substitute Aspas scored from point-blank range shortly after it when Morata's header hit the bar and bounced down to him.
Aspas repaid the favour three minutes later, teeing up Morata for his second with a cleverly disguised pass, before netting again himself with a fierce drive inside the near post.
Liechtenstein's Maximilian Goppel flicked Spain forward Gerard Deulofeu's low cross past his own goalkeeper in the 89th minute to end a miserable night for the home side.
(Editing by Ken Ferris)
Share
