Jenson Button's lap time from the opening day of the Jerez test - when the track was still dirty - left the opposition wary of the MP4-28's potential, and was in contrast to the difficulties Mercedes faced over the week.
Although Hakkinen admits that the next test at Barcelona will deliver a better indication of form, he thinks he has seen enough to judge how the McLaren and Mercedes battle will go.
"To be honest the first test of the season is always a little bit of a gamble," he told BBC Radio 5 Live.
"You don't get the 100 per cent picture of the performance of the car, so it is a little bit early to say.
"I have a strong feeling McLaren is in top form this year. Like we saw in testing in Jerez, the Mercedes was not exactly up to the speed I expected them to be, so I don't see them competing too much this year."
Hakkinen's comments are in contrast to those of his former McLaren team-mate David Coulthard, who reckons Hamilton can deliver from the off.
Speaking in an interview for the Australian Grand Prix podcast, Coulthard said: "Mercedes have a great engine and there are a lot of talented people there, so if they get the aero package right there's no reason why Lewis can't win multiple races and the championship.
"Mercedes have had a couple of years without success but they have a lot of history when it comes to building race cars and they've had time to look at what the other teams are doing and move in the right direction."
Coulthard also pointed towards Hamilton's record compared to his new team-mate Nico Rosberg as a reason for the Englishman to be optimistic this season.
"Nico won the race in China last year, but if you look back over Lewis's performance in the junior formulae compared to Nico's then everything would seem to suggest that he has that little extra spark.
"They've had very parallel careers all the way from karting but Lewis has always been the one that's shone out of the two.
"If Nico can win a grand prix there's no reason why Lewis can't win a race, or even multiple races."