Mercedes and Ferrari face a testing Saturday at the Japanese GP after heavy rain severely curtailed running at Suzuka in Friday practice.
After Sebastian Vettel topped Practice One for Ferrari, Mercedes' Lewis Hamilton was quickest in the afternoon session but only five drivers set a time following a 45-minute delay.
It means Mercedes are still not certain they have found a fix for their struggles in Singapore and Malaysia while Ferrari will only fit the gearbox they believe was undamaged in Vettel's collision with Lance Stroll at Sepang for a definitive test when Practice Three begins on Saturday morning.
Friday's downpour has in any case greatly reduced all of the teams' understanding about their cars' optimal set-up heading into the weekend.
"We didn't even try to learn anything," said Kimi Raikkonen.
But an optimistic Hamilton reported: "The car was a lot better this morning in the dry than it was in the last race. But Ferrari had some impressive pace. We're strong and hopefully we can compete with them.
"The car feels back to normal, so l am ready to race."
But there was a fresh setback for Hamilton's Mercedes team-mate Valtteri Bottas on Friday night when an unscheduled gearbox change was made, triggering a five-place grid penalty.
No panic at Ferrari
Ferrari endured a torrid September, suffering a double first-lap DNF in Singapore and then race-wrecking engine problems in Malaysia.
But Vettel is hopeful Ferrari will be back to their best this weekend.
Japanese GP Practice: Vettel ahead of Hamilton on rain-hit Friday
When's the Japanese GP on Sky?
Rosberg joins Sky F1 in Japan
"The results were not the best the last couple of races but I think we are strong so we have all reason to be confident. And I'm pretty sure on Sunday we will be able to show what we can do," he said.
"If it stays wet then we need to improve because the last time it was wet was Monza and we were not very strong. Malaysia was better but I hope we've learned our lessons. The car is quick so it is up to us to put it in the right place."
The three engine issues in Malaysia which affected both Vettel and Kimi Raikkonen prompted Ferrari president Sergio Marchionne to say there would be organisational changes in the quality department.
But Vettel insists there is no panic at the team after their poor results.
"I'm not sure it's put in the right context," he said. "Whatever happens on track happens on track. What happened in Singapore I don't think you can take much action. In Malaysia we had a problem affecting me and Kimi.
"It's normal you try and understand things and have a shift in people. Knowing what's going on internally, there is no panic or big plans as a reaction."
FIA unlikely to reopen Vettel-Stroll probe
Vettel currently trails championship leader Hamilton by 34 points with five races remaining.
"We are behind in the championship so we need to score much more than them. How we achieve it doesn't matter as long as we achieve it," said Vettel.
"It is much more straightforward if we get our optimum and ideally win a lot of races. I still have a good chance to be world champion."
Mercedes hope for better weekend
Hamilton's pace on Friday offered encouragement to Mercedes after two grands prix in which they have had the third fastest car.
In addition to being fastest in P2, he was just two tenths of a second behind Vettel in Practice One.
Hamilton told reporters in Malaysia his W08 car contained a "fundamental flaw" while on Thursday he described it as "stubborn".
Mercedes technical chief James Allison acknowledged the car could be "difficult", adding "we would like a car that is easier to throw at the race track. We hope next year we make something with a slightly sweeter temperament."
But team boss Toto Wolff was optimistic of a better weekend in Japan, telling Sky F1: "Malaysia was a combination of factors that didn't particularly suit us - the heat being the most important one.
"We have a good car and a lot of downforce but it's a bit of a capricious car - not easy to set up with the tyres. But it's going to be better tomorrow."