Having been drawn in the same opening group stage together, Vandoorne and Frijns had logged exactly the same time to get through to the duels, hinting at a close encounter between the two.
Vandoorne kept it clean in the final, as Frijns endured a heart-in-mouth moment as he stopped just in time to make it around the second corner - miraculously losing only a smidgen of time to the Belgian.
But Frijns continued to look considerably more wayward in the following clutch of corners, meaning Vandoorne was able to keep his nose ahead to beat his rival by 0.35s.
Vandoorne's semi-final against Antonio Felix da Costa was an incredibly close bout and, no sooner had the Belgian looked on the cusp of a break against the DS Techeetah driver after the second sector, he took the Turn 12-13 chicane aggressively and lost his margin - with da Costa marginally ahead.
But Vandoorne held his nerve and kept it clean in the final array of 90-degree corners - sitting a mere 0.015s to the good over da Costa to stamp his card into the final.
Da Costa was the second Techeetah that Vandoorne had to brush aside on his road to the final; Jean-Eric Vergne had looked rapid in the lead-up to his quarter-final battle, but was unable to get dialled into the lap and was bested by four tenths of a second by Vandoorne.
Frijns, meanwhile, beat Andre Lotterer in his quarter-final duel - the Porsche driver relieved to make it into the top eight places after missing part of the second practice session having sustained damage at the chicane.
That set up an all-Dutch semi-final between Frijns and Nyck de Vries, who had seen off Group A topper Jake Dennis in their quarter-final duel as the Avalanche Andretti driver struggled to get his Michelin tyres into the correct window during his outlap.
De Vries' semi-final time was good enough for third over da Costa, while Vergne set the quickest time among the drivers eliminated in the quarters and will start fifth ahead of Dennis.
Pascal Wehrlein was beaten in his quarter-final by da Costa, and starts seventh ahead of team-mate Lotterer in an all-Porsche fourth row.
Among the group stages, championship leader Edoardo Mortara and Jaguar duo Mitch Evans and Sam Bird were dumped out in the Group A qualifying session, with the former pair unable to improve in their final laps as Bird missed the chequered flag by mere seconds.
Wehrlein eclipsed Evans on the top of the timesheets in the final flurry of laps at the end of the session, before being shuffled aside by Frijns, who was devilishly quick in the first pair of sectors to post a 1m40.128s.
Vandoorne then set an identical time to Frijns, but Dennis then fired in a 1m40.069s to head the session to ensure Andretti got one of its cars into the duels as his team-mate Oliver Askew was only 10th fastest in the group.
Lucas di Grassi was unable to prevent both Venturi drivers from being eliminated in the groups, having missed out on the top four by three tenths.
Oliver Rowland's Turn 10 spin produced a yellow flag towards the end of the session, limiting the number of drivers who were able to improve.
De Vries was first to the final flurry of laps in Group B, avoiding any late drama by booking his place in the top four despite a heavy lock-up into Turn 7 - which the Dutchman was able to overcome and coax his Mercedes into the corner.
Lotterer then continued his string of duels appearances with third in the group, before the two Techeetahs swept to the top to assert their pace; Vergne was slightly hampered in the final sector after initially going three tenths up in the final lap, but da Costa then overhauled his team-mate to lead the second group session.
Nick Cassidy was unable to get into the quarter-finals despite initially showing promise, while Sergio Sette Camara was on for a surprise top-four placing before a glacial final sector meant he was only able to post the 10th-best time.
Evans hence starts from ninth ahead of Rowland to complete the top 10 on Saturday's grid, with Mortara starting ahead of Venturi team-mate di Grassi.
Bird was furious with Sebastien Buemi following his elimination, blaming his inability to beat the chequered flag on the Nissan e.dams driver and slapping the Swiss driver's helmet with his gloves as the cars made it into the pitlane.
Rome E-Prix - starting grid:
| Cla | # | Driver | Team | Time | Gap |
|---|---|---|---|---|---|
| 1 | 5 |
 Stoffel Vandoorne |  Mercedes | ||
| 2 | 4 |
 Robin Frijns |  Virgin Racing | ||
| 3 | 17 |
Nyck de Vries | Mercedes | ||
| 4 | 13 |
 Antonio Felix da Costa |  Techeetah | ||
| 5 | 25 |
Jean-Eric Vergne | Techeetah | ||
| 6 | 27 |
Jake Dennis |  Andretti Autosport | ||
| 7 | 94 |
Pascal Wehrlein | Porsche Team | ||
| 8 | 36 |
Andre Lotterer | Porsche Team | ||
| 9 | 9 |
 Mitch Evans | Jaguar Racing | ||
| 10 | 30 |
Oliver Rowland |  Mahindra Racing | ||
| 11 | 48 |
 Edoardo Mortara |  Venturi | ||
| 12 | 11 |
 Lucas di Grassi | Venturi | ||
| 13 | 10 |
Sam Bird | Jaguar Racing | ||
| 14 | 37 |
Nick Cassidy | Virgin Racing | ||
| 15 | 29 |
Alexander Sims | Mahindra Racing | ||
| 16 | 22 |
Maximilian Gunther | DAMS | ||
| 17 | 23 |
Sébastien Buemi | DAMS | ||
| 18 | 3 |
Oliver Turvey | NIO Formula E Team | ||
| 19 | 28 |
Oliver Askew | Andretti Autosport | ||
| 20 | 7 |
Sergio Sette Camara | Dragon Racing | ||
| 21 | 33 |
Dan Ticktum | NIO Formula E Team | ||
| 22 | 99 |
 Antonio Giovinazzi | Dragon Racing | ||
| View full results | |||||
