Lewis Hamilton rode his luck for the second race in a row to stretch his world championship lead over Sebastian Vettel but said he fears a Ferrari backlash in Japan this week.
After finishing second in the Malaysian Grand Prix, Hamilton was asked by reporters if Ferrari's pace was a big concern. "Yeah, definitely," he said.
"We have some work to do," he added before flying to Japan to prepare for Sunday's race at Suzuka.
"These next races are going to be crucial in terms of ironing out some of the creases we have with the car."
Hamilton took his lead to 34 points after finishing behind Red Bull's Max Verstappen at the final running of the Malaysian Grand Prix at Sepang on Sunday.
But the drive of the day was from the flying Vettel who carved his way from the back of the grid to finish fourth.
Hamilton's Mercedes also struggled for raw pace in Singapore a fortnight ago.
But there he took a surprise win from fifth on the grid after avoiding the first-lap crash that took out both Ferraris and Verstappen.
"We cannot be distracted by the fact we got lucky again this weekend," said Mercedes team principal Toto Wolff.
"Benefiting from Ferrari's problems shouldn't hide that they have caught up 30 seconds this race," he said.
"It was clear after just a few laps that both Red Bull and Ferrari had better cars than us today."
Hamilton could be handed another break in Suzuka as Vettel may incur a grid penalty if he needs a new gearbox after a crazy post-race shunt with Lance Stroll.
The incident wrecked the rear of the German's car and he will be demoted five places on the grid in Japan if the gearbox needs changing.
In Malaysia an engine failure saw Vettel fail to set a qualifying time, condemning him to the back of the grid despite Ferrari having been fastest in practice.
"It was a much better weekend than we had anticipated after our Friday. Considering the issues we did have with the car this is a good result," conceded Hamilton.
Mercedes have dominated Formula One for the past four years but it seems their era of supremacy could be coming to an end as both Ferrari and Red Bull appear to have the better car.
"After a weekend where we have lacked a significant chunk of performance, we couldn't have imagined such a good outcome, but we can't let that distract us from the job we have in front of us," said Wolff.
"We leave Malaysia with a lot of question marks and we need to find answers to them in the next days."