The Roja boss believes his side found it tough to contend with the hot and humid conditions in Fortaleza against Nigeria on Sunday
La Roja looked a little laboured in the final third until Fernando Torres netted shortly after coming on as a second-half substitute to settle Spanish nerves, and Del Bosque believes both their opponents and the heat proved formidable over the 90 minutes.
"It was a tough match," Del Bosque told reporters.
"We had a great opponent and they managed to create a balance in possession, looking for victory. We had control, patience and I think this was an unfair result as both teams should have scored more.
"To have a balance between patience and all-out speed is very beneficial. We had two big opponents today, the heat and Nigeria. Playing 90 minutes in this heat and at this rhythm is impossible."
Roberto Soldado was restored to the starting line-up at the expense of Torres and David Villa, who both netted hat-tricks against Tahiti, but Del Bosque was coy on who would start the semi-final against Italy on Thursday.
"Roberto has done his job and Fernando has done his," he continued.
"A team needs fresh legs. Soldado had a couple of good plays where he was in front of the goalkeeper. We're happy with both [players]."
Jordi Alba was another key player in la Roja's victory, contributing two goals following trademark bursts down the left-hand side, and his coach could not help but applaud his efforts.
"Our system makes it so that players like Jordi Alba can appear in great offensive situations," he explained.
"We didn't lose our balance and had Jordi, who is a splendid player, creating chances.
"I'm more about players than a system of play. Every team needs to have offensive organisation and defensive organisation. We never lost defensive capabilities but we've played a bit more offensively in the second half. In the second half, we defended better."