Mercedes F1 boss Toto Wolff expects the new season to be "a three-way fight between us, Ferrari and Red Bull".
He said, "Our drivers seem reasonably happy with our vehicle, but it remains to be seen how well it performs driven in anger".
Mercedes are aiming for a fifth consecutive title double.
"Our reliability in testing looked good but we need to be careful to draw any conclusions from that - despite getting some good mileage with the new auto in Barcelona, many of its components have not even come close to the life they need to complete during the season".
Hamilton said, "I personally feel stronger (than last year)". "Melbourne in 2017 was a combination of my errors and some misfortune, I crashed in qualifying and then we had a technical issue for the race".
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Lewis Hamilton has thanked "all those have supported me all this way" as he prepares to begin the defence of his Formula One world title.
"We are still working on some things for the first race and it's not really possible to make any predictions based on the performance of the other teams, because everyone is running a different program", he said. That's a positive. I don't know where we are in terms of the pack but reliability has been fantastic and the auto feels great.
"I have a lot of confidence in our team, and I can't wait to be in Australia, because once we get on track there, we will all be driving and racing under the same conditions".
He wrote on Instagram: 'As I embark on my journey to Australia, I think of all those have supported me all this way, who still support me to this day.
Hamilton's team-mate this year is Finn Valtteri Bottas, who hopes to provide a tougher challenge for the Briton than last year as he seeks to convince the team to extend his contract into 2019. Mercedes's reigning world champion and Ferrari's Vettel can both join the legendary Juan Manuel Fangio as five-time world title-holders this year, second only to Michael Schumacher's seven.
"The best thing I think that could happen in 2018 is for Ferrari to perform up to its true potential and to take on the competition", he said last month after the company's fourth-quarter financial results. That's it; it's completely up to me. Vettel swept home in last year's opening race against Hamilton in Melbourne before the British ace went on to land the championship with two races to spare from the German. "I'm super-confident in my team and the work they've done, I just think it's going to be close". "That will be possibly the key".