Lewis Hamilton says Mercedes is battling "plenty of problems", admitting the engine reliability dramas that have affected the Formula 1 team during pre-season testing are a concern.
Mercedes has had to swap engines three times during pre-season testing, following a failure for Valtteri Bottas in last week's first test and an oil pressure anomaly for Hamilton on Thursday.
The issues, allied to problems for customer outfit Williams, have left Mercedes with the task of getting on top of the reliability issues before the season opener in Melbourne next month.
Reflecting on how the team's pre-season preparations had gone, Hamilton said that although the potential of the car was clear, the team was still not entirely happy with its programme.
"I think we've had a good winter test," he said.
"But it's not been perfect, we've found that we've got plenty of problems that we are trying to iron out.
"I don't know how long it will take to iron out but that is never a bad thing necessarily to discover through testing.
"But otherwise, I think our performance has been quite good in the sense of laps we've got, the mileage, apart from yesterday [Thursday], and the processes and things we discovered along the way.
"I've no idea where we stand compared to the others, the team will probably have a better idea of that."
Asked if the engine troubles were a worry, Hamilton said: "Is it a concern? Yeah. For sure.
"Normally in the pre-season we have much more confidence in the reliability so it has not been perfect. We are on our second or third engine.
"So it is definitely not an easy or relaxed scenario for us.
"But I have every confidence in the guys back at the factory that they can do the best job they can in the next two weeks to make sure we start on the right foot."
Beyond the reliability issues, Hamilton said he felt comfortable in the W11.
"I don't know where everyone else is, but I understand the car well," he said.
"I am comfortable and confident in terms of getting a car in Melbourne and knowing that I can attack and extract the most from it.
"But if that's going to be enough to be ahead of the others we will find out."