The defending champions advanced to the quarterfinals of the European Championship on Monday, getting an 88th-minute goal from substitute Jesus Navas in a 1-0 win over Croatia.
Spain dominated possession as expected but failed to create many clear opportunities at the Arena Gdansk. And on the other end, Spain goalkeeper Iker Casillas needed to make a pair of clutch saves in the second half to bail out the world champions.
"It was a question of believing in what we are doing," said Spain midfielder Andres Iniesta, who flicked a short pass to Navas for the deciding goal.
Spain, which is trying to win its third straight major title, will next face the second-place team in Group D on Saturday in Donetsk, Ukraine. Italy also advanced from Group C after beating Ireland 2-0.
"We made a great step forward. We qualified. That's what motivates us for the quarterfinal. Our philosophy and style of play are good and we proved it in today's match," Del Bosque said. "The team played well even though the match didn't go as we planned."
After wasting a number of late chances, Spain finally broke open the Croatia defence when substitute Cesc Fabregas lobbed a through ball for Iniesta. The Barcelona midfielder then passed to Navas, who put his shot into the roof of the empty net.
"We played to the end and we deserved the win," said Navas, who scored his second career goal for Spain.
Croatia coach Slaven Bilic, who is stepping down from the team following its elimination, wasn't convinced that Spain can win the European title again.
"I don't see them as big favourites," Bilic said. "There are some other teams that have more pace and they are more hungry to win and maybe more aggressive on the pitch than Spain team."
Navas' goal came moments after Iniesta and Fabregas failed to convert late opportunities, with the European champions often trying to string together one too many passes.
"We suffered a lot tonight. They are very good," Iniesta said. "We had to be patient. We had to wait for our chance. That's what we did in the end."
Croatia could have reached the quarterfinals with a victory — or a draw depending on the score. Casillas, however, made a reflex save on Ivan Rakitic's header in the 59th minute and then punched out substitute Ivan Perisic's low volley in the 79th.
Casillas, who recorded his 76th shutout in his 134th appearance, was Spain's steadiest player in defence as the backline was often troubled in the second half by balls into the box.
Spain appeared to be more hesitant than against Ireland, but more attacking than against Italy, which held it to a 1-1 draw.
The Spanish defence also did well to shut down Croatia striker Mario Mandzukic, who scored three goals in the team's first two matches.
Croatia: Stipe Pletikosa; Ivan Strinic, Vedran Corluka, Gordon Schildenfeld, Domagoj Vida (Nikica Jelavic, 66); Danijel Pranjic, Ivan Rakitic, Ognjen Vukojevic (Eduardo, 81), Luka Modric, Darijo Srna (Ivan Perisic, 66); Mario Mandzukic.
Spain: Iker Casillas; Sergio Ramos, Gerard Pique, Jordi Alba, Alvaro Arbeloa; Sergio Busquets, Xabi Alonso, Xavi Hernandez (Alvaro Negredo, 89), Andres Iniesta, David Silva (Cesc Fabregas, 73); Fernando Torres (Jesus Navas, 61).