With just over six weeks out from the Cup holders' tournament opener against the Netherlands in Salvador, Negredo has not started in City's past two league outings.
Manager Manuel Pellegrini turned to Edin Dzeko over Negredo, with the Bosnian scoring early in their win over Crystal Palace on Sunday.
Negredo admitted his form had not been up to scratch, with his last goals in a City shirt coming in the League Cup semi-final win over West Ham in January.
"I was playing games when I wasn't fully fit – but things have turned around now and I am fit again," he said.
"I am not featuring as regularly as before. I turned it around earlier in the season so I have got to have the same mentality to turn things around once again.
"I did that by working hard on the training field and scoring goals when I got my chance. Hopefully, that's what will happen again.
"It's just a case of working as hard as I can to get as many minutes on the pitch as possible to repay their faith in me."
Negredo last featured for Spain as a second-half substitute in their 1-0 friendly loss to South Africa in Johannesburg in November.
But despite a lack of attention from national boss Vicente Del Bosque, Negredo remains hopeful of boarding the plane to South America.
"It's my dream to be involved in the World Cup – it would be great and is something I am fighting for," he said.
"Not playing regularly here means the national manager has not seen much of me – but I won't give up. Quite the contrary – I am full of optimism.
"I believe he will still have confidence in me. I will carry on working hard in training and score as many goals as possible.
"It's about carrying on with the same dynamic that served me so well earlier in the season."
Negredo said his ability to find his way into City's first XI earlier in the league campaign showed he can earn his spot from lower down the food chain.
"I settled in (at City) right from the word go although initially my time on the pitch was limited," he said.
"But I continued to work hard and when I got the chance I started to score goals.
"I was able to bring something to the party in terms of my performances."
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