The New York City race one victor pipped his team-mate to the top of the times, with Guenther’s 1m26.838s lap ensuring he was the only driver to dip under the 1m27s barrier.
Only Oliver Turvey had failed to lap within a tenth of free practice one pacesetter Jean-Eric Vergne’s Friday benchmark, but on Saturday morning the field was hugely spread out.
Valencia race-two winner Dennis was some 0.375s adrift of his stablemate’s best effort, despite using four 35kW attack mode boosts to Guenther’s one in a bid to close the gap.
It was then left to Porsche driver Andre Lotterer to complete the top three, after he ran provisionally fastest but was bumped back in the final minute to fall 0.629s in arrears.
The forecasted rain duly arrived in time for the session, with drivers frequently aquaplaning on the lesser-used perimeter circuit roads that make up the outside portion of the circuit.
Three minutes into the half-hour run, Mercedes driver Stoffel Vandoorne was caught out by the standing water and shunted into the wall on the approach to the Turn 10 right-handed hairpin.
He ripped the front wing assembly off his Silver Arrows 02 and skated on to the run-off, where he could not revive the car to return to the pitlane for repairs.
That meant he failed to set a time and was forced to watch the remainder of the session from the wall, and it follows on from his two punctures in New York City that massively derailed the team’s strategy in the US as he was forced to heavily limit running and fit scrubbed tyres.
Stoffel Vandoorne, Mercedes Benz EQ, EQ Silver Arrow 02
Photo by: Sam Bagnall / Motorsport Images
Envision Virgin Racing driver Robin Frijns was another to be caught out, with the Dutch racer crumpling the nose of his car when he skated sideways down the track at Turn 6.
Frijns had been managing oversteer through the preceding corners, but then lost the rear as the track sloped downhill and skidded parallel to the concrete wall before eventually making contact.
He was able to rejoin from the run-off after a minor brush with the TecPro barrier, eventually running to only the 22nd fastest time.
Sergio Sette Camara soon followed suit down at Turn 6, losing the rear and sliding into the run-off area that had only just been vacated by Frijns.
The Dragon Penske Autosport driver would, however, post the fifth quickest time behind Mahindra Racing driver Alex Lynn.
The second Mercedes of Nyck de Vries ran to sixth place ahead of Rene Rast and the two Nissan e.dams, with Sebastien Buemi eclipsing his team-mate Oliver Rowland.
Edoardo Mortara rounded out the top 10, while Mitch Evans led points leader and Jaguar Racing team-mate Sam Bird in 11th place.
Sam Bird, Jaguar Racing
Photo by: Simon Galloway / Motorsport Images
The DS Techeetahs would wind up two seconds off the pace, reigning champion Antonio Felix da Costa clocking 18th as team-mate Jean-Eric Vergne was 19th.
Lucas di Grassi was also forced to abandon the session early when he made a rare mistake aboard his Audi machine at Turn 9 and clonked the inside wall and damaged his suspension.
He was 23rd, beating only Vandoorne and finishing behind Nick Cassidy and Frijns as Audi powertrains occupied positions 21st to 23rd.