Speaking to reporters on the wet and cold final day of the second pre-season test at Barcelona's Circuit de Catalunya, the 2008 world champion dampened any excessive optimism about his prospects.
"I think that people are talking us up at the moment," said the Briton, who was top of the timesheets on a day of frequent interruptions and red flags when times were largely meaningless due to the conditions.
Jenson Button, his McLaren team mate last season, was second quickest.
Hamilton disagreed with the assessments of Red Bull's triple world champion Vettel and Ferrari's Alonso, who had spoken of him and Mercedes as title contenders.
"I really don't see that happening at the moment," Hamilton added while pointing out that Mercedes, who won one race last season with Germany's Nico Rosberg, were more than a second and sometimes two off the pace in 2012.
"We haven't caught two seconds up and the other teams would have put another second on over the winter, so we've not gained three seconds. That's just a fact," he said.
"Hopefully by the end of the year, we'll have gained three seconds - that's our goal. But definitely not at the beginning."
Hamilton won four races last year with McLaren but has said before that it will be tough to triumph this season and his sights are more on 2014 when the engine rules change.
He said on Friday that he did not want to have any race wins "gifted" to the team because others had tripped up. "We want to do it through hard work," he declared.
Despite the subdued comments, Mercedes have been doing plenty of laps in Barcelona and Rosberg was the fastest driver on the timesheets on Tuesday. Hamilton did a hefty 121 laps on Wednesday alone.
Over the eight days of testing so far, eight different drivers from four teams - McLaren, Mercedes, Ferrari and Lotus - have set the fastest times. The final four-day test is in Barcelona next week.
Champions Red Bull, chasing their fourth successive drivers' and constructors' titles with Vettel, have yet to set a fastest lap in any session but still look the team to beat even if all could be closer than ever.
The season starts in Australia on March 17. Last year kicked off with seven different winners in the first seven races.
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