Ferrari, Renault and newcomers Honda begin the chase as reigning champion bids to defend its titles.
Andy Cowell, the man credited with Mercedes' 2014 power output advantage in Formula One, believes its opposition has more scope to improve this year – and will inevitably catch up as a result.
Following a dominant 2014, when Mercedes won both the drivers' and constructors' championships aided by a power advantage estimated at 60bhp, in an interview with Germany's Auto Motor und Sport on Thursday Cowell said he expects rivals to gain ground.
"The rules of physics and chemistry are the same for us all," said Cowell. "I see no reason why the competition cannot build an engine as good as ours.
"Of course we do not stand still, but the higher you are in your development, the flatter the curve is to improve," he told the German publication.
Rivals working hard
Ferrari set the test pace at Jerez, and its new engine is believed to be a major step forward in terms of its power unit and electrical energy harvesting.
Renault has also worked hard on its structure and engine, but was hampered by an ERS issue at Jerez, while newcomer Honda is struggling with its early installation.
Cowell also said Mercedes' rivals have the advantage of being able to see exactly where the German giant is strong.
"Thanks to GPS measurements, they know the areas they need to look at, so Ferrari and Renault just have to aim higher," he said. "That is their advantage.
"Honda is a bit of an unknown, but we should not underestimate them. We will take a big step forward, but I cannot guarantee that it will be enough to be better than the competition."
1000bhp possible with these engines
Cowell also commented on Bernie Ecclestone's desire for 1,000bhp engines, even though the new turbo V6 era is only a single season old.
"I believe that with the current architecture, we can achieve that (1,000bhp), and produce more sound, if we increase the fuel flow rate," he said.
"However, we should not dilute the concept of energy efficiency. Technology transfer from motorsport to the road industry is important for Formula One. It's why Honda came back."