The forward tucked away a penalty in his first game for his country since returning from a broken leg, but it was his Barcelona team-mate Pedro who stole the show with two goals
Santi Cazorla opened the scoring and Pedro doubled the home side's advantage soon after. Substitute Xavi netted a third two minutes into the second half before Barcelona team-mate David Villa announced his return to the international arena by converting a penalty shortly after entering the fray.
Pedro then put the seal on an impressive individual display when he tucked away a terrific through-ball from David Silva with 17 minutes remaining.
The reigning world and two-time European champions, who take their first step on the road to Rio against Georgia next week, fielded an experimental starting lineup, with Xavi and Andres Iniesta among those named on the bench. However, La Roja still had far too much class for a game but limited Saudi Arabia side.
Indeed, the only surprise was that it took so long for them to break the deadlock, although that was primarily down to Fernando Torres' wayward finishing, the Chelsea man, who was making his 100th appearances for his country, wasting two fine through-balls inside the opening 10 minutes.
As it was, it was left to another member of Spain's Premier League contingent, Cazorla, to open the scoring, the Arsenal schemer tapping home from close range after Waleed Abdullah could only parry a shot from Pedro.
The latter made no mistake five minutes later, finding the back of the net with a delightful chip just moments after Nasser Al-Shamrani had gone close with a header for the visitors.
Cazorla, who had teed up Pedro for the second, then very nearly conjured a third goal for the home side but his sublime curling effort cruelly came back off the woodwork.
Still, Spain did put the outcome beyond all doubt when Xavi, who had replaced Cesc Fabregas during the interval, found the top corner with a free-kick that saw the ball not only find its way past Abdullah - but also through a hole in the net.
The game now won, La Roja boss decided to make more changes and substitute Villa, who has only just recovered from the horrific leg break he suffered last December, made it 4-0 from the penalty spot, after Sayyaf Al-Beeshi had felled Monreal in the area.
Pedro then heaped further misery on the beleaguered Saudis when he latched onto a defence-splitting pass from David Silva before coolly dinking the ball over Abdullah, putting himself in real contention for a starting berth in Tuesday's Group I clash with Georgia in Tbilisi.