Mercedes suffered a set-back on the first day of the final pre-season test at the Circuit de Catalunya after an MGU-K issue forced it to miss the entire afternoon session.

Hamilton had been in fine form early in the day, setting an impressive 1min 24. 881secs lap time on medium tyres, a benchmark that was enough to see him end the first morning comfortably out front.

However, the Mercedes W06 would not come back out on track thereafter, with Mercedes saying it was working on fixing a problem with the Hybrid system. This was later diagnosed as an MGU-K failure and brought about an early conclusion to the day.

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It means Hamilton ends the day with just 48 laps under his belt, though this is the first major issue suffered by Mercedes since the start of the testing and it has still covered a significant more miles than other teams. Nico Rosberg will step into the car tomorrow.