Four-times Formula One world champion Lewis Hamilton sat out the second day of pre-season testing on Tuesday after Mercedes said they wanted to make the most of good weather with rain threatening.
The Briton had been due to take over from Finnish team-mate Valtteri Bottas in the afternoon at the Circuit de Catalunya but Mercedes said a decision had been made to continue with one driver.
The usual break for lunch was abandoned.
"This is to maximise our mileage after the poor track conditions limited running this morning," Mercedes said on Twitter.
"Mileage is king in #F1Testing, so we want to make the most of the current conditions. A driver swap would require switching out seat, pedals, even stickers when we could be out on track."
Hamilton completed only 25 laps on a cold and wet Monday afternoon, with snow later in the evening and temperatures plummeting.
"It`s great to be back in the car after three months off, but unfortunately we didn`t get the chance to complete many laps today owing to the bad weather," he had said on Monday evening.
"The track was very cold in the afternoon, meaning it was tough to get any sort of heat into the tyres. The tyres weren`t really working in these temperatures, so I could not get a real feel for them.
"It`s hard for me to say a whole lot about the car because I did not drive it for a long time, but the initial feeling is good."
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.
Hamilton missed the final day of the first test last year, making clear that time that he was not a big fan of wet weather tyres.
"I might fake a pulled muscle in the morning and let Valtteri do it," he had said then.
Bottas was top of the Tuesday lunchtime timesheets, with 43 laps under his belt and a best time of one minute 20.270 seconds.
McLaren had a better morning than Monday, when Fernando Alonso`s car lost a wheel and crashed into the gravel, with Belgian Stoffel Vandoorne second fastest and with 32 laps to his credit.