Hakkinen, who won the 1998 and 1999 world championship while driving for McLaren, reckons Mercedes's early testing form doesn't stand out.
"To be honest the first test of the season is always a little bit of a gamble," he told BBC Radio 5 Live, referring to last week's opening test in Jerez.
"You don't get the 100 per cent picture of the performance of the car, so it is a little bit early to say."
But the Finn nonetheless suggests that McLaren, who will field Jenson Button and Sergio Perez this season, are on top, with Mercedes a little further behind.
"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."