The German took the lead of the race with 10 laps remaining, having circulated inside the top three throughout and built enough of a lead to retain the top position after taking his final Attack Mode.
Having managed the pace at the head of the field, Guenther came under attack from Rowland on the final lap with the Nissan driver making three notable attempts to overtake but failing to move ahead.
Polesitter Rowland had led for the majority of the 35-lap race, having held the position off the line despite being forced to defend from Edoardo Mortarawho had jumped Guenther from the cleaner and grippier side of the grid.
Concerning drivers that overtaking around the tight and twisty layout would be difficult to prove to be correct, which meant track position was crucial, and in stark contrast to other races, Attack Mode activations took place much later in proceedings.
The leading positions remained unchanged for much of the opening segment, but when Mortara was forced to defend into the Turn 6 right-hander from Guenther on lap 10, the small break allowed Rowland to take his first of two Attack Modes the following lap and retain the lead.
Mortara also took his extra boost of power on the same lap, promoting Guenther up to a position before he found himself in the lead when Rowland took his second Attack Mode on lap 13.
Guenther dropped down to third on lap 14 after taking his first Attack Mode, but immediately moved back into second after passing Mortara on the run into Turn 16.
Thereafter, Guenther shadowed Rowland, and when the Nissan driver lifted off to save energy on the run into the Turn 10/11 chicane, Guenther took a lead he was never to relinquish.
Behind, reigning champion Jake Dennis finished third after an overtake on Antonio Felix da Costa three laps from the end when the Porsche driver lost momentum after an optimistic move around the outside of Rowland into Turn 15.
The Portuguese driver retained fourth at the flag ahead of team-mate Pascal Wehrleinwho had lost ground after running side by side with Dennis through the final sequence of corners, which pushed him back two positions mid-race.
Mortara had dropped to sixth at the flag after taking his final Attack Mode just five laps from the end, but he was disqualified post-race for energy over usage.
It promoted Abt’s Nico Muller to sixth, with Jaguar’s Nick Cassidy taking seventh having recovered from a starting position of 19th.
It was a difficult day for team-mate Mitch Evansas the Kiwi finished 14th after stopping for a new front wing following contact with Robin Frijns into Turn 6, the debris bringing out a safety car as the race was extended by two laps.
Frijns was classified seventh, having also lost his front wing after contact with Norman Natowith the Andretti driver handed a five-second penalty that dropped him out of the points from seventh.
Sergio Sette Camara (ERT) and the second Nissan of Sacha Fenestraz completed the top 10.
