Jenson Button has admitted that improvements to his McLaren car will not transform it into a potential winner in China this weekend.
McLaren have taken new rear bodywork in an attempt to improve the pace of the car that proved a disappointment at the start of the season.
Button said: "It's not a step that's going to suddenly send us to the front. We need to keep working and we will.
"There is an improvement with the parts but not as much as we would like."
Button was sixth fastest in both Friday practice sessions in China but said that was not an accurate representation of McLaren's form.
"No," he said. "It was different people in front of us each time and I think people just run different fuel loads.
"I feel we're getting the best out of the car but we don't have as much pace as we'd like, which is the same for everyone when you compare them to Mercedes."
Asked if he was suggesting that the team had a problem with the data they were being given by wind-tunnel simulations not matching that from the track, Button said the new parts had not been tried out in the wind tunnel before the team committed to taking them to China, as would normally be standard practice.
"But we feel they are better for all-round downforce and consistency around the lap," he added.
Sporting director Sam Michael admitted that some of the new parts McLaren were using in China were a reaction to the team's poor form and some had been planned before the first race.
Button added: "The issues with the car are still the same issues with the car. We need to sit down and look at the data. There is a lot to look through.
"It does feel a little bit better in the areas where we're struggling but it's not a big leap forward."
Despite his initial remarks about Mercedes' pace, when pressed Button said it was hard to be sure from Friday's running which teams were in the best shape for Sunday's grand prix because the top teams had shown different form on the two tyre options, of the 'soft' 'option' tyre and 'medium' prime tyre.
"I don't know if [Mercedes] put in more fuel to run the option but they obviously weren't as competitive and Lotus and Ferrari were very competitive but they weren't so much on the prime," Button said.
"From first practice you'd say Mercedes would walk this grand prix, but now when you look at both tyres it is more difficult to understand who is strongest at the moment."
Button said the big problem for all the teams in the race would be the excessive performance drop-off of the soft tyre, which is as much as 1.5 seconds a lap quicker than the medium initially.
Button said: "It's unbelievably quick for one lap, but that's it. It's done after that and the degradation is massive.
"You can't look after it or change the balance to help it, it just happens.
"You're going to see pit stops very early in the race and spending most of the race on the prime.
"It's very tricky and I don't think you're going to see many quick laps in the race. I think we're all going to be given a time to go to and that's it."
Button's team-mate Sergio Perez had two light crashes - one in the entry to the pit lane - and admitted he was struggling with the car.
Perez said: "We don't really understand what happened: I had a lot of wear, the front tyres were losing temperature and as soon as I hit the brakes I locked the tyres, [but I was] not pushing at that point.
"I'm not very confident with the car and that makes things difficult.
"We have so many question marks, we have to analyse everything that we have done because we don't know which direction we are going. The car is so inconsistent for us."