Sky Sports F1's Martin Brundle has described Mercedes' correlation problems as "very worrying" after a disappointing Japanese Grand Prix.
Mercedes had their worst result at Suzuka in over a decade as George Russell finished in seventh with Lewis Hamilton two places behind in ninth.
Hamilton is yet to finish higher than sixth in the opening four events this year, which is a first in his 17-year F1 career.
Mercedes initially attempted a one-stop race after the red flag for Daniel Ricciardo and Alex Albon's crash at Suzuka, but they went backwards and did not have good enough tyre wear.
The team have admitted the numbers and results they see in the wind tunnel have not correlated onto the track.
"They've got to understand this car and I think that's a grave concern for all of the people there," said Brundle on the latest episode of the Sky Sports F1 Podcast.
"There's a lot of very clever people, with a huge amount of resource, performance tools and budget. I'm not going to try and second guess what's wrong with it, or state what I think is wrong with it, because if they don't know, then I certainly don't know."
Mercedes are fourth in the Constructors' Championship behind Red Bull, Ferrari and McLaren. Lance Stroll's deficit to team-mate Fernando Alonso means Mercedes are ahead of Aston Martin, but only by one point.
In Japan, Hamilton was fifth in first practice and declared the session as the best of Mercedes' season, stating the car was in "more of a sweeter spot".
Just 48 hours later, the seven-time world champion said the car was "pretty bad" during the Grand Prix itself, which Brundle highlighted.
"They cannot get a handle on these ground-effect cars. This is the third season of these regulations. They turn up, they think they have aced it, a lot of positive noises, and then it still bounces a little bit with the porpoising," he continued.
"But their problem is, from time to time, the thing performs beautifully and they are really quite fast in phases. But, they can't seem to reproduce that session to session, let alone day to day, let alone Grand Prix to Grand Prix.
"This is the problem they have got - this knife edge of a car that sometimes looks like they have finally sorted it and more of the time they just can't understand it.
"When you've got that, when all of your tools and all your clever people don't correlate with the stopwatch and the performance of other cars on the track, and you can't seem to nail it down, then that's really frustrating and I would say very worrying."
Chandhok: Wolff is frustrated
Toto Wolff told Sky Sports F1 that Mercedes' first stint on the hard tyres at the Japanese Grand Prix was "atrocious".
However, he also said "the car is becoming quicker" and the performance was "much better" than the final result looked.
Karun Chandhok was slightly puzzled by the comments from the Mercedes team principal.
"I think he's a man who's frustrated. The best Mercedes was 45 seconds off the win but if we take the Red Bulls out of it, they were still 25 seconds behind the best non-Red Bull, which is a huge amount in a Grand Prix," said Chandhok.
"Toto made an interesting comment to Ted (Kravitz) after the race, where he said the first stint was terrible and after that, it was not so bad.
"I looked back at some of the numbers and it's little bit better, but in the first stint, they lost 11 seconds to Sainz in 15 laps, which is three-quarters of a second nearly.
"After that, they still lost on average about 0.6 per lap. So it was a little bit better, but not a lot better. So I think they were the comments of a frustrated man, who has come off a tremendous run of success and is again staring at another season, of the team that he co-owns, unlikely to win the World Championship.
"That's not the standards that he's used to or expects, I think, from the team."
Listen to the full episode of the Sky Sports F1 Podcast with Martin Brundle and Karun Chandhok, where they talk Silly Season, the 2026 regulations and the first Sprint race in China. Subscribe now on: Apple Podcasts | Spotify | Spreaker
Next up is the return of the Chinese Grand Prix on April 19-21, which is also the first Sprint weekend of the season. You can watch every session live on Sky Sports F1 and steam every F1 race and more with a NOW Sports Month Membership - No contract, cancel anytime
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