Mercedes was caught by surprise by Ferrari's 2015 resurgence, according to 1997 world champion Jacques Villeneuve.
In Australia, the reigning world champion team appeared to have maintained or even extended its dominance of 2014 by easily driving to a one-two.
But then in Malaysia, Ferrari's Sebastian Vettel won on merit.
"It seems that, having achieved success, the team (Mercedes) relaxed a little bit," Villeneuve told France's Auto Hebdo.
"I think Mercedes did not believe anyone could catch up with them, not realising that the situation had in fact changed."
Pirelli's F1 chief Paul Hembery, however, said Mercedes was not alone in being caught unaware by the level of Ferrari's resurgence.
"It was definitely a surprise to any F1 expert," he told Finland's Turun Sanomat newspaper.
"You couldn't sense it in the winter tests or even in Australia," Hembery added. "By Malaysia I think Mercedes realised they would have to work a little harder."
Whether German Vettel has a red car good enough to challenge for the title, however, remains unclear.
So it may be left up to the on-form world champion Lewis Hamilton's teammate Nico Rosberg to up his game and put some more spice into the 2015 title battle.
"Victory would help him return to his previous form," said Villeneuve, "so Nico needs to start winning as soon as possible."
Williams' Felipe Massa, however, says he would put his money firmly on Hamilton.
"Last year it was difficult to be sure (who would win)," the Brazilian told UOL Esporte, "but this year Hamilton is stronger in every way than Rosberg.
"We have seen a few squabbles but mostly on the psychological side," Massa added, "which shows that Rosberg is suffering more than Hamilton.
"F1 is 100 per cent about the detail," he said.
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