Carlos Sainz Jr says he probably wouldn't be racing in F1 if he had failed in his and Red Bull's objective for him to win the 2014 Formula Renault 3.5 Championship comprehensively.

The Spaniard will form one half of an all-rookie line-up at Scuderia Toro Rosso in 2015 alongside Max Verstappen after winning the 2014 FR3.5 Championship with seven rave victories over 17 races.

Indeed, Sainz believes his eventual title win in FR3.5 was critical for him in his quest to make it to F1 and to continue with Red Bull backing having failed to make the impression he wanted in Formula 3 and GP3 in the seasons leading up to Formula Renault 3.5.

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Blaming a lack of consistency in his performances, Sainz - having been placed in FR3.5 with the sole target of winning the title - says the season was about proving he could 'put everything together' to form a title bid.

"I saw it as simply, I wouldn't have gone into F1 without putting all my things together," he told "The FR3.5 year was to confirm myself as a complete driver, such as being able to fight back in tricky situations, being able to be consistent in the whole year, do all of the pole positions and the race wins.

"Helping to consolidate myself and arrive to Formula 1 with more confidence, with a clearer picture about how I work and how I feel.

"If I wanted to have just a small chance of getting in to F1, the only small chance by getting it was by winning a championship. Coming second was absolutely not an option and I knew that since I came into 3.5 it's quite tough to realise that you can only win, and it was tough moments. It was tough, but in the end I became strong. Coming second, I can say that I wouldn't be here in Barcelona testing."

Despite his title-winning success, Sainz's place in F1 wasn't assured for several months after Red Bull opted to sign up Max Verstappen unexpectedly, leaving his future undecided. Indeed, Sainz admits he was 'disappointed' to see Verstappen get an F1 seat over him.

"To be honest, it was a big disappointment. I was doing things perfectly. After doing things really well, I had a perfect beginning of the season. When they announced Max, you don't feel good. You feel a bit down and you feel like your opportunity has gone.

"I kept the faith though and I picked myself up. More than ever, I just tried to continue demonstrating to Red Bull that I still deserve that seat and it was a help for me to just do another step forward in my work, my motivation to get that seat and in the end by luck or coincidence, Daniil could upgrade to Red Bull and I sit up here. Whether it is work ethic and perseverance or coincidence, I don't know. Maybe it's a bit of both."