Mercedes have had things all their own way in the last three Formula One seasons but start the two-day test in Bahrain on Tuesday with plenty to consider.
Ferrari have won two out of three races so far this season to lead the constructors' championship and it is Sebastian Vettel, not Lewis Hamilton, who tops the drivers' standings.
"We have a bit to do at the tests," Hamilton said before his return on the Sakhir Circuit which hosted the Bahrain GP on Sunday. "We need to improve the car and our knowledge about the tyres so we can be in a better position in the next race."
Tyre provider Pirelli have altered the dynamic of the product delivered to all F1 teams this season and the early indication is Ferrari have adapted better than Mercedes.
Mercedes' pole-sitter Valtteri Bottas struggled for pace with a tyre pressure issue on Sunday while it is accepted Ferrari have a general advantage in high temperatures - where the relationship between car and tyre alters.
Hamilton's start on Tuesday was hampered by a cable plug reportedly falling out and halting his car after just a few laps. It is the sort of minor thing which, in a tight race with Ferrari, could have a major impact on the championship.
Ahead of the next Russian Grand Prix in Sochi on April 30, the Mercedes chiefs are also considering whether they can continue to let their drivers race freely in light of the increased external competition.
Hamilton and reigning world champion Nico Rosberg, now retired, fought hammer and tongs for titles but that was in seasons when Mercedes were largely unchallenged.
Bottas, in three races to date as Rosberg's replacement, does not seem to be at the level required to chase the title and at one point in Bahrain was holding up the quicker Hamilton.
Eventually the decision was made to prioritise Hamilton but he could not make up the gap to leader Vettel and eventually finished second.
"We don't like that," Mercedes motorsport chief Toto Wolff said. "It is not what we have done the past couple of years. But the situation is different now.
"We'd like to give each of them equal opportunity at the start of the race. We owe it to them. Then you see what we did in the race. We made the call. We made the call twice."
While Hamilton, and Mercedes, were working on 2017, Vettel was already testing the 2018 tyres on Tuesday before returning to the present rubber on Wednesday. Such is the advantage of being the world championship leader.
By Jens Marx
