Sebastian Vettel has described Lewis Hamilton and Mercedes as one of the strongest team/driver combinations Formula 1 has seen.
As a pairing, Hamilton and Mercedes have claimed the drivers' and constructors' titles in six of the last seven years.
The only championship to slip through Hamilton's fingers was in 2016 when then team-mate Nico Rosberg beat him to it, although Mercedes still took the honours in the constructors' battle.
Vettel joined Ferrari in 2015 from Red Bull, moving after the first year of the new V6 turbo-hybrid engine regulations had marked Mercedes as the benchmark team.
Although the German put up a valiant fight in both the 2017 and '18 seasons he fell short as Hamilton sealed the title in Mexico with two races remaining on both occasions.
"We were up against a very strong team/driver combination, one of the strongest we’ve seen so far, but our goal was to be stronger than that and in this regard, we failed," said Vettel, who has since joined Aston Martin.
"We had good races, bad races, we got close, sometimes far away, there are a lot of reasons why, but in the big picture it’s not unfair, it’s just the truth."
Since joining Mercedes in 2013, Hamilton has scored 74 race wins for the team.
When securing his seventh world title at the Turkish GP last year, Hamilton also took his 73rd win for Mercedes, surpassing the record held by Michael Schumacher for the most wins with a single team - 72 with Ferrari between 1996 and 2006.
Asked if criticism of his six-year partnership with Ferrari was unfair, given he ranks below only Schumacher  and Niki Lauda  in the number of races wins scored for the manufacturer with 14, he added: "It still doesn’t change anything. We still failed.
"We had the ambition and targets to win the championship and we didn’t. I think it’s just an honest reflection. I don’t think saying it out loud changed anything."
In joining Aston Martin for the upcoming season, Vettel will race alongside Canadian Lance Stroll.