Christian Horner believes Red Bull's recovery from such a tough start to 2014 has been one of the team's biggest achievements.

Red Bull struggled with a lack of running throughout pre-season testing, with Renault's new power unit uncompetitive as well as unreliable. However, the team fought back to finish a clear second in the constructors' championship behind Mercedes, and Horner told Crash.net during an exclusive interview that the recovery was one of Red Bull's best.

"I think it's been the most challenging year," Horner said. "As a team, if you look back over the last six years I think it's actually been one of our biggest achievements to come back from where we were pre-season.

"Almost having done no mileage whatsoever, to be at the end of the season second in the world championship, to be the only team to have beaten Mercedes, to have won three grands prix and have however many podiums as we've had I think has been - considering where we started and our horsepower handicap - an enormous achievement by the entire team."

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And Horner said Red Bull never seriously considered changing power unit supplier despite the tough start to the season with Renault.

"Well ... not really. It's very simple. We have a very long-standing relationship with Renault, we've won a lot of races with them. Mercedes would never give us an engine, Ferrari you can only ever be a customer there and it would be wrong to forget in a short space of time all the good we've achieved over the previous four or five seasons with Renault.

"So what we've chosen to do is the harder route but to get more embedded, more involved in the organisation. Both Viry and Milton Keynes now are much closer together."