Monaco – Mercedes took a wrong turn in Monaco Grand Prix practice on Thursday with Lewis Hamilton only eighth fastest after a difficult afternoon around the narrow street circuit.
The triple world champion had been fastest in the morning session but was more than a second off the pace in the afternoon. Hamilton's Finnish team-mate Valtteri Bottas was 10th in the later practice.
"We took a wrong junction in setting up the car in a direction which we believed was good," said Austrian team boss Toto Wolff.
"We couldn't back out of it. We committed to the set-up as it takes longer than a session to change and we decided to push through and collect some data," he told Sky Sports television.
Mercedes has won in Monaco for the past four years with Hamilton the 2016 winner, but Ferrari's Sebastian Vettel looked significantly quicker on Thursday as the only driver to lap inside one minute 13 seconds.
Vettel sets fastest Monaco lap ever
The German's effort was the fastest lap ever of the classic circuit that winds up through Casino Square, down through a hairpin into a tunnel and then sends cars blasting past the ranks of yachts moored in Monaco harbour.
Wolff said the car had looked competitive in the morning and Mercedes would have to backtrack.
"That's not a very difficult exercise but we lost a session where we could have progressed," he said. "We are lacking data so it's about sticking our heads together, staying calm and trying to work as well as possible on Saturday morning."
Hamilton, who is hoping for a 65th career pole position on Saturday to equal the tally of his late boyhood idol Ayrton Senna, said the tyres just weren't working in the required range.
Describing it as a "night and day" difference from one session to the other, he said he did not fully understand what had gone wrong.
"This afternoon was a bit of a struggle for us but it did get better. Practice Two I struggled with a bit of grip and wasn't really able to extract the most from the tyres for some reason. "We've got some work to do, that's for sure...I have no idea whether we will be on the pace on Saturday."
Monaco Grand Prix: Second Free Practice
1. Sebastian Vettel (Germany) Ferrari 1:12.720
2. Daniel Ricciardo (Australia) Red Bull – TAG Heuer 1:13.207
3. Kimi Raikkonen (Finland) Ferrari 1:13.283
4. Daniil Kvyat (Russia) Toro Rosso – Renault 1:13.331
5. Carlos Sainz Jr (Spain) Toro Rosso – Renault 1:13.400
6. Max Verstappen (Netherlands) Red Bull – TAG Heuer 1:13.486
7. Sergio Perez (Mexico) Force India – Mercedes 1:13.799
8. Lewis Hamilton (Britain) Mercedes 1:13.873
9. Kevin Magnussen (Denmark) Haas – Ferrari 1:13.890
10. Valtteri Bottas (Finland) Mercedes 1:13.902
11. Stoffel Vandoorne (Belgium) McLaren 1:13.946
12. Jenson Button (Britain) McLaren 1:13.981
13. Felipe Massa (Brazil) Williams-Mercedes 1:14.003
14. Romain Grosjean (France) Haas – Ferrari 1:14.022
15. Esteban Ocon (France) Force India – Mercedes 1:14.093
16. Lance Stroll (Canada) Williams-Mercedes 1:14.474
17. Nico Huelkenberg (Germany) Renault 1:14.870
18. Jolyon Palmer (Britain) Renault 1:15.616
19. Marcus Ericsson (Sweden) Sauber – Ferrari 1:15.691
20. Pascal Wehrlein (Germany) Sauber – Ferrari 1:15.695