His boss has labelled 2015 “pretty much of a write-off” after just five Formula One races, but Daniel Ricciardo remains stubbornly optimistic.

The Australian star is still hopeful Red Bull can close in on their Mercedes and Ferrari rivals, despite failing to finish any higher than sixth this season.

Ricciardo – a three-time winner last year – placed seventh in Sunday’s Spanish Grand Prix, ahead of his Russian teammate Daniil Kvyat in 15th.

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“It is not enjoyable being in a grand prix like today when you are just going around, you’re not racing,” team boss Christian Horner said.

“We want to get back. The frustration is that not all of it is in our hands. We are relying on our partner (engine supplier Renault).

“Daniel drove a strong race today, and did a great job to extract everything he could out of the car to finish seventh – which is where we are for the moment.”

The 25-year-old has already used all four allocated engines for the season; Kvyat has used three.

Ricciardo faces grid penalties if he uses any more.

“This year is pretty much of a write-off, you have just got to go for it, even if you end up using 20 engines,” Horner said.

“It is far easier to make a fast engine reliable than a reliable car fast.”

Even after a slew of updates to his RB11, Ricciardo reckons seventh was the “maximum” he could get out of the car around Circuit de Barcelona-Catalunya.

“There were some parts of the race that were encouraging but we’re still further off than where we want to be, but we’ll keep pushing,” he said.

“We came here with upgrades this weekend but they didn’t give us what we were expecting, so that’s something we’ll keep working on.

“We’re still a fair bit behind Williams at the moment but I think we have a good chance to close the gap a bit further in Monaco.”

Despite acknowledging Renault had taken a step forward in terms of reliability, with both their cars and the two Toro Rosso entries finishing the Spanish GP, Horner said the focus must now be on improving performance.

Red Bull, who swept the championship between 2010 to 2013, are currently fourth in the constructors’ standings – trailing Williams by 51 points, second-placed Ferrari by 102, and frontrunners Mercedes by 172.

“The step we need to get on the back of that group can’t be done just with the chassis,” said Horner.

The next race is in Monaco, Ricciardo’s home city, on May 24.

– AAP