The Maserati MSG racer swept to the fastest lap in all timed sessions, as per his efforts on Tuesday, but ultimately settled on his best – a 1m25.449s time – at the end of the day to beat Stoffel Vandoorne to the punch.
Wednesday’s running consisted of a race-day dress rehearsal, featuring a half-hour practice session prior to a loose qualifying run-through and a mock race session.
Guenther, picking up from the excellent form he displayed on the opening day, had once again had his morning porridge and fired his Maserati MSG machinery to a 1m26.178s at the close of the session to shuffle DS Penske's Vandoorne away from the top of the charts.
This preceded a qualifying group run-through, which suffered a brief intermission as the McLaren of Rene Rast came to a halt at the Turn 13-14-15 chicane installed after the traditional Circuit Ricardo Tormo final corner.
A series of loosely formatted qualifying duels followed, although the definition of duel was tested when Jean-Eric Vergne began the running on his lonesome.
Although Guenther had headed the nominal group qualifying stage, he was beaten by Vandoorne by 0.074s in their mock qualifying final duel - the first time the German had ended a session off the top.
The two were then offered another pair of opportunities to practice duels, where Guenther prevailed in both - and logged a 1m25.492s in the last run to set the first lap of the test faster than last year's running with the Gen2 car.
For the race simulation, the order was effectively set on a first-come-first-served basis, and thus McLaren had beaten its rivals in the race to the pit exit.
Rast led Hughes through the opening half of the allotted 24-lap duration, which featured a safety car ahead of the half-distance mark. On the restart, however, Hughes moved past his team-mate to bag the lead, while Guenther collected third from Jean-Eric Vergne.
The Maserati MSG driver continued to display his comfort with the Gen3 car and later nipped past Rast for second. Looking to continue a near clean sweep of sessions throughout the test so far, Guenther then looked to slash the gap to Hughes.
However, the rookie Briton was able to retain his advantage despite Guenther's best efforts - and crossed the line 1.654s clear.
By the end, Rast was 6.5s shy of his countryman, but ahead of a close-knit group of Vergne, Pascal Wehrlein, Edoardo Mortara, Antonio Felix da Costa and Vandoorne. Lucas di Grassi and Oliver Rowland completed the top 10.
Jake Dennis, Mitch Evans and Nico Muller were the only remaining drivers on the 'lead lap' as Norman Nato, Sam Bird, Sebastien Buemi and Sacha Fenestraz all spent time in the pits.
Andre Lotterer, Robin Frijns, Nick Cassidy and Dan Ticktum 'retired' from the race simulation, while Sergio Sette Camara did not start the session.
A two-hour test session succeeded the race simulation, but there were relatively few takers in the opening half-hour; Cassidy and Frijns produced some exploratory laps, as did Sette Camara after missing the early afternoon action.
The Brazilian shot to the top with a 1m28.213s, some way away from Guenther's earlier benchmark at 350kW power, but his lap was later chalked off for track limits.
Taking to the track became a much more popular pursuit thereafter, however, as teams looked to tie up their run plans for the day; Vandoorne was able to flex his muscles and fire his black-and-gold chariot to the top of the pile with a 1m26.336s in a more representative showing, ahead of Nato and Evans as the first hour drew to a close.
Bird then ended up off the road at the end of that first hour at Turn 4 to put a pin in running, as the marshals responded by reaching for the red flag.
Nato then deposed Vandoorne from the top of the timesheets by 0.003s, but Guenther once again raised the bar with a 1m25.686s - a shade slower than his earlier benchmark from the qualifying test.
He then got within 0.002s of that earlier best, logging a 1m25.494s in the final half-hour as Vandoorne joined him in the 1m25s ballpark - albeit just over 0.2s adrift.
With less than five minutes to go, Guenther underlined his grasp of the test with a tour clocking in at 1m25.449s, picking up the gauntlet and throwing it down again – but his call went unanswered.
Vandoorne ended the session clear of Nato and Vergne, as Mortara ensured a second Maserati MSG factored in the top five.
