The German set the quickest time of the morning's running as the ABB FIA Formula E World Championship's new era kicked off in earnest with the first laps of the all-new Gen3 car.
His late 1m26.096s was enough to pip rookie Jake Hughes to top spot by 0.082s; the NEOM McLaren driver impressing in the first official timed outing for the Gen3, following his off-season switch from Formula 2 to the team in papaya. Guenther enjoys testing in Valencia, having topped the times here twice before.
Another Brit in Mahindra Racing's Oliver Rowland followed with Edoardo Mortara - the third placed driver in Season 8's final standings - made it two Maseratis in the top four on its return to top tier single seat motorsport after six decades away - an encouraging start for the first Italian team to line up on a Formula E grid who had actually been one-two midway through the session.
Hello, Gen3 😍— ABB FIA Formula E World Championship (@FIAFormulaE) December 13, 2022
Reigning champion Stoffel Vandoorne set the fifth fastest time of the morning - though four-tenths down on Guenther's benchmark, while Antonio Felix da Costa rounded out the top six on his bow for Porsche.
The session was hit by rain, making conditions tricky. Mitch Evans beached his Jaguar I-TYPE 6 in a spin on the exit of Turn 5. Earlier on, Envision Racing's Nick Cassidy did likewise, with a 360 at Turn 6 throwing him into the gravel trap.
As always at testing, being fastest isn't always the priority, with a lot of work to be done between now and close of play on Friday afternoon.
Each of the championship's outfits has been flat-out since Season 8 and the Gen2 era drew to a close in Seoul, with each firmly focussed on getting the most out of the Gen3, which rings in Formula E's biggest technological leap yet, with the cars the fastest, lightest, and most sustainable electric race car ever built.
Teams are working tirelessly to extract the maximum from an all-new platform for Season 9 with a 200mph (280kph) top speed, a 60kg lighter, smaller chassis, an additional front powertrain and no rear brakes plus a power output 100kW up on Gen2 at 350kw with double the regen capacity - meaning more than 40% of the energy used in-race is via regeneration under braking.
Plenty to learn, then, and the Gen3 looks properly sharp firing out of the chicane onto the start/finish straight!
We're back at 14:00 CET with the second session of the day. Follow every lap at fiaformulae.com/live.