Red Bull surprised by its early-2016 form

After six races of the season Red Bull just nine points behind second-placed Ferrari in the constructors' championship, and 76 adrift of Mercedes, having already won a race with Max Verstappen in the Spanish Grand Prix.

Significant improvements by engine supplier Renault have played a key role in the upturn in form, and at weekend's Canadian Grand Prix both Daniel Ricciardo and Verstappen will be running the upgraded engine only the former had in Monaco.

"We didn't expect to be anywhere near as competitive as we have been in the first part of the year," Horner told Autosport.

"In pre-season we felt we would be more competitive over the second half of the year, and I still believe that will be the case.

"What's surprised us is how competitive we have been since Melbourne, and the results we've had, notably in Barcelona where we beat Ferrari fair and square.

"So I don't think anybody could have expected the start to the year we've had, but we are definitely in the ascendancy, you can feel that, that buzz, within the whole factory."

Renault managing director Cyril Abiteboul said after Verstappen's win in Spain he felt it was the first of many podiums this year for Red Bull.

Ricciardo followed up with second place in Monaco, although the Australian should have claimed victory but for a pitstop error from the team.

Horner is wary, however, of suggesting his team can push Mercedes for the titles this year.

"Our challenge is to try and close the gap to Mercedes, to take on the Ferraris when and where we can," added Horner.

"To have achieved a victory already at this point in the year is certainly way beyond our expectations.

"It would perhaps be getting slightly ahead of ourselves to suggest we can push Mercedes in the championship given it the dominance it has enjoyed, even this year.

"But the target is obviously to close that gap down and attack them race by race.

"We've clearly a good engine now, and the chassis is just so strong, which is down to a great technical that has done a great job so far."