Looking repeatedly behind him in the technical finale in Diekirch, Vansevenant crossed the line 18 seconds ahead of closest pursuer Davide Formolo (Movistar), with a Van der Poel-led group blasting across the line at 41 seconds.
Race officials later disqualified Formolo from the race for using the banned 'super tuck' position on a descent.
"I already did the Vuelta a España [this autumn] so I'm not feeling super-fresh, but when the engine is running it doesn't stop anymore," Vansevenant said afterwards.
"When I'm alone I can keep going. I knew I had some power left, but to win this way is really nice," Vansevenant said.
"I had a feeling I was the engine of the two [with Formolo] and I wanted to be sure that I didn't give it away, so I went full-full-full on the last climb."
In an ultra-hilly stage with nearly 4,000 metres of vertical climbing, Vansevenant made his winning attack four kilometres from the finish, shedding Formolo and moving ahead for his first victory since the toughest stage of the 2023 Tour of Oman.
Van der Poel closed down multiple attacks, most notably by UAE Team Emirates Marc Hirschi, but with two stages remaining, Vansevenant has now opened a 34 gap overall on the leading favourite and the race remains wide open.
On a great day, all around for Soudal-QuickStep, an early dig by Vansevenant's teammate Louis Vervaeke and Johannes Kulset (Uno-X Mobility) proved more successful than expected, too, staying away for nearly 180 kilometres despite Vervaeke's later reservations that he would have preferred a bigger break than one with just two riders to form.
The duo's gap nonetheless became so large at one point that Kulset, 6:13 down on Van der Poel on GC, briefly became provisional race leader on the road, even if the 20-year-old Norwegian did struggle notably on the brutally steep mid-stage Um Knupp climb.
Then when reinforcements in the shape of counter-attackers Bastien Tronchon (Decathlon-AG2R La Mondiale) and Lidl-Trek's Amanuel Ghebreigzabhier arrived a little later, the quartet managed to maintain their advantage all the way to a difficult finishing circuit, tackled twice and featuring five classified climbs in just over 30 kilometres.
UAE Team Emirates' determination to work in the chasing bunch was quickly explained when Felix Großschartner charged away on the initial ascent of three of the Montée de Haemerich, catching the flagging foursome just after the first time across the finish line in the small town of Diekirch.
The Austrian went straight to the head of the break, and Kulset immediately paid the price for the surge in pace, while stage 2 winner Mads Pedersen (Lidl-Trek) followed suit in the crumbling bunch and dropped back as well.
An acceleration by Andreas Kron (Lotto-Dstny) sparked yet more testing moves in the peloton and so many challenges forced a prematurely isolated race leader Van der Poel to start bridging gaps in person.
In turn, Van der Poel's repeated clampdowns caused a loose regrouping of some three dozen riders at the front of the race as it moved along a ridge road at speed. But even the in-form Dutch champion had too much on his hands to stop the ever-recognisable figure of Vansevenant from going clear with 24 kilometres to go, though, just before the second key climb, the punchy Seitert. And he was equally powerless when Formolo, often in form on home soil in the Italian autumn Classics, quickly joined Vansevenant.
Formolo and Vansevenant were still hanging on as they crossed through the finish line for the second last time, but despite a gap of less than a minute on a very volatile chase group powered by UAE and Van der Poel, their collaboration was limited at best. Vansevenant repeatedly testing a flagging Formolo did not do him any favours with the Italian, either - at one point Formolo patted the Belgian on the back with some ironic congratulations after he had been dropped yet again. That said, collaboration in the main pack, now almost a minute back, was crumbling too.
The stage was long decided in favour of the two breakaways, but when Marc Hirschi (UAE Team Emirates) made a counter-move around eight kilometres from the finish, Van der Poel flashed across in no time at all, realising that a concerted chase was the only way to have any chance of winning overall. By steadying the ship and bringing home a group of 13 chasers for third and four potentially invaluable bonus seconds Van der Poel remains very much in contention - though dropping down to third overall means he has a fight on his hands for sure
Ahead, meanwhile, Vansevenant's last surge in speed at the foot of the Haemerich proved too much for Formolo and after clearing the top of the steep, wooded climb one last time, the Belgian climber picked his way carefully down the drop into the finishing town to claim the third victory of his career. The 25-year-old Soudal-QuickStep racer claimed the overall lead too, but although he seemed far from certain of his chances of keeping it in Saturday's critical 15.5-kilometre time trial against such a formidable opponent as Van der Poel, he insisted he would not give it up without a fight come what may.
"I will go full for sure, but it's not my specialty so I'm not sure I can keep the [leader's] jersey," Vansevenant recognised.
"Our race mission is already completed, though, as now we have a stage victory and I am feeling very motivated. So I will try my best to defend it."
Alasdair Fotheringham has been reporting on cycling since 1991. He has covered every Tour de France since 1992 bar one, as well as numerous other bike races of all shapes and sizes, ranging from the Olympic Games in 2008 to the now sadly defunct Subida a Urkiola hill climb in Spain. As well as working for Cyclingnews, he has also written for The Independent, The Guardian, ProCycling, The Express and Reuters.
