A power cut halted Formula One's pre-season testing in Bahrain on Wednesday with Ferrari's Charles Leclerc top of the afternoon timesheets after Mercedes rookie Andrea Kimi Antonelli set the morning pace.
The track action was red-flagged, forcing the drivers to pit, after teams lost electricity during the late afternoon under a darkening sky and with the Sakhir circuit floodlights turned off.
Engineers were seen using torches while team garages were plunged into darkness, although some back-up generators appeared to be working. The media centre wifi and lighting went out.
"I came to the pits and said 'it's dark out here. I need to change my visor'. Then I realised there are no floodlights around," Mercedes' George Russell told Sky Sports television.
"It would have been hectic if this was an hour later and the sun had already set."
Seven-times world champion Lewis Hamilton had already done 70 laps, more than a full race distance, and was fifth fastest in 1:31.834 on his first day of testing for Ferrari since leaving Mercedes, although the times meant little.
Leclerc then took over the car after lunch and set a 1:30.878 best.
Hamilton's replacement Antonelli, 18, was the first on track and led after the opening four-hour stint with a quickest lap of one minute 31.428 on medium tyres and 78 laps under his belt without incident.
He then handed over to Russell, who was second fastest when the power went and ahead of McLaren's Lando Norris.
New Zealander Liam Lawson, four-times world champion Max Verstappen's new Red Bull teammate, spun his new RB21 in the morning but was second fastest then with Williams' Alex Albon third.
Lawson did 58 laps and Albon 63 on a day that started breezy and chilly at around 14 degrees celsius.
Racing Bulls' Yuki Tsunoda, fourth on the timesheets, matched Antonelli's 78 lap tally, with British rookie Oliver Bearman completing 72 for Haas.
Australian rookie Jack Doohan was testing for Alpine, and ended the session sixth with 68 laps and compatriot Oscar Piastri was eighth for champions McLaren and did 66 laps.
Teams have three days of testing in Bahrain before the 24-race season starts in Australia on 16 March.
Meanwhile, F1's prestigious Monaco Grand Prix will have two mandatory pitstops per driver to liven up the racing, the sport's governing body said on Wednesday.
The 'strategic overhaul', doubling the current minimum, was approved by the FIA's World Motor Sport Council at its first meeting of the year.
"Following recent discussions in the F1 Commission, a specific requirement for the Monaco GP has been approved mandating the use of at least three sets of tyres in the race," the FIA said.
A minimum of two different tyre compounds will have to be used if the race is dry.
Overtaking is extremely difficult around the Mediterranean principality, with qualifying and pole position even more crucial than at other races.
"Monaco is super-exciting on a Saturday, for us drivers it's incredible, it’s the best qualifying of the year," Ferrari's Monegasque driver Leclerc said last week after the measure was discussed by the F1 Commission.
"Then on the Sunday it can get a little bit... there's not much happening. So I think this is a way to spice things up a bit more."
