Although Nico Rosberg topped both Thursday sessions in Monaco, the Mercedes driver admits over a long distance he could struggle.
Rosberg was the driver to beat in Thursday's practices, setting the pace on the soft tyres in the morning and later in the afternoon on the supersofts.
The German posted a 1:14.759 to edge his team-mate Lewis Hamilton by 0.315s while the Ferrari drivers were four- and five-tenths down.
But despite his flying pace, Rosberg, who claimed pole in Bahrain and Spain, concedes it's only Thursday and a lot can still change.
"I am feeling comfortable in the car. It has been going well today, but you really do not know what other people are doing with fuel," Rosberg told Sky Sports News.
"You just have to wait and see and I have to do my job and try to improve. For sure we are somewhere in the hunt at the front, but I don't know if it will be enough."
And even if Mercedes do get the front of Saturday, their long distance pace has let them down too often for them to feel comfortable.
Although the team achieved three successive pole positions leading up the Monaco GP, they have yet to win a grand prix and in fact only Hamilton managed to turn his pole into a podium result.
As such, Rosberg admits he has concerns.
"There is the problem of race pace, which is still going to be crucial here to have a great result.
"It is still our difficulty and that is what we have been working on today. We now understand a few things but there are still a couple of question marks which we need to solve as soon as possible.
"There will be very few stops in the race - two stop or one stop. So you really have to try and get the degradation as good as possible and that will really help to have a successful race."
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