Ferrari's Sebastian Vettel was fastest in Formula One's pre-season testing at a chilly Circuit de Catalunya on Tuesday while Mercedes rival Lewis Hamilton took one for the team and "sacrificed" his scheduled stint.
Vettel, like Hamilton a four times champion, lapped with a best time of one minute 19.673 seconds on soft tyres and also completed the most laps (98) despite flurries of snow later in the day.
The German's fastest time was quicker than that posted by any team at the first Barcelona test of 2017 but he said Ferrari were focusing on reliability more than performance at this stage.
"From an operational point of view, everything went smoothly and I’d say the car is working. But today’s conditions are not the ones in which you’d normally run an F1 car," said Vettel.
Hamilton had been supposed to test in the afternoon but Mercedes changed the schedule to keep Finnish team mate Valtteri Bottas in the car all day rather than wasting time with a handover.
The usual lunchtime break was cancelled.
"With the weather conditions being up and down, and the loss of track time we had yesterday, it made sense to keep Valtteri in the car this afternoon," said the Briton in a team review of the day's action.
"Having run in the morning, he was comfortable in the car already and we would have lost about an hour of running time with the driver switch," added the champion.
"With no real understanding of whether good weather was coming or not, I took the decision to sacrifice that time in order for the team to gain better understanding of the car by going through the run plan."
Bottas was second overall, 0.303 slower than Vettel, with his best time also set on softs.
"I don't think I've ever driven in such cold conditions before in Formula One," said the Finn. "The tyres are just not made for these conditions."
McLaren had a better morning than on Monday, when Fernando Alonso's car lost a wheel and crashed into the gravel, with Belgian Stoffel Vandoorne third fastest but with only 37 laps to his name.
The team said a broken exhaust clip had led to hot gases being blown back onto a wiring loom which included the brake wire. McLaren stripped the car down but the weather had deteriorated by the time it was ready again.
Red Bull's Max Verstappen also had technical problems, after Australian team mate Daniel Ricciardo had set the fastest time on Monday, with the car kept in the garage for most of the morning as mechanics worked on it.
The 20-year-old Dutch driver ended up fourth fastest.
The forecast for Wednesday is wet and teams have been considering adding an extra day to testing to compensate for the missed track time.
Williams have a private filming session scheduled for the circuit on Friday so any extra day would probably have to be added next week.