Ferrari design chief James Allison has insisted that Kimi Raikkonen made his Ferrari teammate Sebastian Vettel sweat at times during the 2015 season.
The Finn was rumoured to be close to losing his seat with the Scuderia midway through the season, but was retained by the Italian team until the end of next year.
However, while Vettel was considered a hit in his debut season for Ferrari, Allison has claimed that the German was made to work for his money by Raikkonen.
He told BBC Sport: "[At Silverstone], Sebastian was utterly candid that he couldn't live with Kimi at various sections of the track in the race, he was not as quick.
"At other times in the year a few errors have crept in and [Raikkonen] has had some real misfortune, where our reliability has dropped the ball for him. But what we have seen through the season, and latterly particularly, is Kimi really making Sebastian work for his money."
Ferrari finished second in the constructors' championship.