Isco said he felt "alive" after his first career hat-trick helped Spain thrash Argentina 6-1 at the Wanda Metropolitano Stadium in Madrid.
The Real Madrid playmaker has seen his club football limited in spells under Zinedine Zidane this season.
However, Isco produced a superb display for La Roja against Argentina, whose captain Lionel Messi again watched from the stands because of fitness concerns.
Spain had drawn 1-1 against Germany last week and have now extended their unbeaten run under coach Julen Lopetegui to 18 matches.
"In Madrid I don't have the confidence a player needs, " Isco said in a post-match interview, as quoted by Spanish media outlet Marca.
"The matches with the national team give me life. Here I have the confidence of the coach.
"I want to show that I'm a good player. Lopetegui shows me confidence with the minutes he gives me.
"Maybe I'm the problem in Madrid. I don't know how to win Zidane's confidence."
After Gonzalo Higuain shot wide early on, Diego Costa fired Spain ahead in the 13th minute, before Isco struck either side of a header from Argentina defender Nicolas Otamendi.
Thiago Alcantara added a fourth, with substitute Iago Aspas also on the scoresheet before Isco netted his third with 16 minutes left.
"Despite the result, this match wasn't easy," Isco added. "They made things difficult for us, they were very aggressive.
"We mustn't forget that this is part of our preparation for the World Cup. The victory can give us a lot."
Spain coach Lopetegui also looked to play down the final scoreline.
"It is an important victory but it does not give points in the World Cup," he said at a press conference, as quoted on the Spain Football Federation website.
"We are happy, but the World Cup has not started yet.
"Another thing is how we manage this euphoria. We know where we are, which is going calmly because we know what a World Cup is about."
Argentina coach Jorge Sampaoli said Messi's absence was due to fatigue.
He said, according to Marca: "Messi trained with us, but then he felt tired and couldn't play.
"He was with the group all the time because he is very involved for the World Cup.
"He has kept close proximity to the national team, helping the new guys out."
On the result, Sampaoli added: "I take the blame for the goals, don't point to the players.
"The difference (between the sides) in the game wasn't as large as the result.
"We have to learn from this. It cannot happen in the World Cup."