Max Verstappen has said Red Bull is still encountering "big problems" with its car after two difficult Friday practice sessions ahead of the Japanese Grand Prix.
Verstappen was 10th fastest in second practice and 1.4 seconds off Oscar Piastri's fastest time at Suzuka on Friday.
"It's not very good to be honest," Verstappen said after the session. "We are lacking balance and grip -- two opposites from FP1 to FP2, and both of them are not very good.
"So from our side a lot of work to be done to understand why we are having these big problems at the moment. Not a good day."
The difficult start to the race weekend in Japan follows a disappointing Chinese Grand Prix two weeks ago in which Verstappen finished ninth in the sprint race and failed to finish the grand prix.
Verstappen said the root cause of the lack of performance was different this weekend, but was still leaving him off the pace of the front-runners.
"We just struggled a lot more with balance of the car, grip," he added. "It was not similar to China, but we're still off.
"We're still not really understanding why we're that far off in Sector 1, in basically medium to high speed a lot. That's something that we need to work on.
"I don't think it's an easy fix overnight. But a few things that are not going right at the moment."
Red Bull's chief engineer, Paul Monaghan, said the team would attempt to make changes to improve ahead of final practice and qualifying .
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"You just have to look at our relative position -- it is not at the standard we set ourselves or the standard Max sets us," Monaghan said.
"I think we've identified a few things that are wrong and now it's a case of can we confirm it and understand it enough that we can influence it, cure it, correct it or whatever for tomorrow, and let's see how tomorrow goes.
"Today I would describe as challenging."
