Mercedes 'incredible' in 2014, says Formula 1 rival Red Bull

The run of four straight drivers' and constructors' titles for Horner's squad was ended in emphatic fashion by Mercedes last season, with world champion Lewis Hamilton and team-mate Nico Rosberg winning a record 16 races between them.

Horner believes Mercedes deserves credit for leading from the front in F1's new era of hybrid turbocharged V6 engines, though it admitted it was braced for its own engine supplier Renault to lag behind the Silver Arrows.

"I think the difference to Mercedes was far bigger than we expected it would be," he said. "We feared that there was going to be a deficit, but nowhere near the scale we saw.

"I think first we need to congratulate Mercedes, who have done a truly incredible job with their power unit.

"I think we were in the worst of all situations with Renault in that we were uncompetitive with an unreliable engine. But credit to them for sorting the reliability out on the engine.

"It's now the performance deficit that we really need to focus on."

Red Bull bounced back from a calamitous pre-season testing programme to win three races in 2014, though all came from new driver Daniel Ricciardo as quadruple champion Sebastian Vettel suffered his first winless campaign since his F1 debut half-season in 2007.

Though Vettel has now left for Ferrari, Horner believes Renault's restructuring and Red Bull's new works team status means the engine battle should be closer this year.

"I think Mercedes invested more strategically than Renault," he added.

"It was something we were aware of and something we pushed Renault on, but unfortunately their management changes came too late to make an impact into this year, but hopefully they will make a big influence in the next few seasons.

"I think the philosophy to embrace Red Bull as the works team and to work as a partner as opposed to a customer was what needed changing, because the power unit needs to be fully integrated."