Kimi Raikkonen posted the fastest time of a wet fourth morning of pre-season testing in Barcelona for Ferrari as an electrical fault brought Lewis Hamilton's week to a premature end.
Three-time defending world drivers' and constructors' champions Mercedes had enjoyed a near faultless first three days despite a series of rules changes allowing for faster, more powerful cars this season.
However, Hamilton didn't get out of the garage with times across the paddock slowed by artificially wet conditions after tanks soaked the Circuit de Catalunya overnight.
"Electrical fault kept us in the garage this morning, so I've decided with the team not to drive today as I wouldn't have learned much," Hamilton posted on his Twitter account.
"Shame not to drive but it's been a great few days. The guys have done an awesome job. Can't wait to be back in the car next week!"
Hamilton's teammate Valtteri Bottas managed just eight laps before lunch and will take charge for the afternoon session.
Raikkonen's best time of 1min 22.305 was over two seconds slower than the mark set by Bottas on Wednesday.
Williams also called an early end to their first test after damage suffered to the FW40 in rookie Lance Stroll's crash on Wednesday.
"A second chassis will be prepared at track this afternoon, as originally planned, with the team aiming to be back on track for the second test next week starting on Tuesday 7 March," Williams said in a statement.