Guenther's late lap was 0.8 seconds up on the fastest time from the race last year of a 1m09.786s. He was followed in close company by teammate Edoardo Mortara, who was a fair way - almost 0.2 seconds, in fact - back in second spot.
Reigning champion Stoffel Vandoorne also did well on the dusty track, firing third to make it a top three of Stellantis powertrains at the top of the timing screens.
Mitch Evans was the strongest of the Jaguar runners in the factory Jaguar TCS Racing I-TYPE 6 but some half-a-second back from Guenther's best.
TAG Heuer Porsche's Pascal Wehrlein is looking to fight back having been deposed from top spot in the standings in Monaco and made a good start with the fifth quickest time, just ahead of the fellow Porsche 99X Electric of Jake Dennis in the customer Avalanche Andretti.
Current Drivers' World Championship leader and form man Nick Cassidy (Envision Racing) finds himself downj in 17th with work to do.
More than just lap times
It's our second trip to Jakarta but our first with the GEN3 and new Hankook tyres. That means no data! Every team is on an equal footing in their preparation and simulation work heading into the race weekend as they have been throughout this season. Round 10 is a blank slate, with Free Practice the only chance teams and drivers will have to gather information and check everything corrolates with their simulations before they get competitive in qualifying ahead of the race. Round 11 on Sunday will be a different story, though, after a whole round's worth of track time under the belt.
Lap times are far from the only focus. Drivers and engineers will be working flat out to understand things like energy management profiles and setup ahead of qualifying and the race later in the day - making sure everything tallies with their simulation work.
On top of energy management, teams work through everything from their systems, setups and ATTACK MODE deployments and launches throughout practice to make sure they're on-point when it counts.
Coverage of Free Practice 2 follows from 09:10 on Saturday 3 June. Qualifying is next up at 10:40 and the 2023 Gulavit Jakarta E-Prix Round 10 itself starts at 15:00.