Timing the unhook from his pilot-fish Lukasz Wisniowski perfectly, Van Poppel launched his bid for victory with around 200 metres to go, leaving behind Mezgec, Sagan and a bunch of podium places from earlier this week.
While the Dutchman said he wasn't really frustrated with his three podiums, sprinters do only race for one thing.
"Today victory was finally mine," Van Poppel said. "That was the key point - the lead out. Everyone was really impressive.
"We took the lead early and even (GC hopeful) Wout did a great pull. Michal and Lukasz did a great job and all I had to do was my own sprint. That was enough."
On the overt right-hand bend at one kilometre to go, a Cannondale-Drapac rider slipped on the wet road, felling some of the bunch behind the sprint favourites. Mezgec rather than Caleb Ewan, got the chance to contest the sprint, but not because of the crash.
Ewan unlatched from the yellow jersey group along the lumpy 130 kilometre route obviously feeling the effort of his win on top of the 240 kilometre raced yesterday.
But Orica-Scott director Dave McPartland was relatively unfazed.
"At first it was a bit of a disappointment not to have (Caleb) there," he said.
"(But) Luka doesn't get that many opportunities to sprint for himself, but he's shown what great form he's in over the past month or so and he was really close to the victory today. We are really pleased."
Tejay Van Garderen (BMC) reminded everyone he's still around by making into the day's break with Maxime Monfort (Lotto-Soudal), Antwan Tolhoek (LottoNL-Jumbo), Moreno Moser (Astana) and Maximilian Schachmann (Quick-Step Floors).
The quintet's lead never reached more than around three minutes and by the final circuit in town with 22 kilometres to go, was down to one minute and 30 seconds, minus Scachmann who abandoned after falling on the slippery roads.
His fellow escapees falling back to the bunch, Van Garderen lashed out in a last ditch effort on the uphill with 12 kilometres to the line. The American dangled out the front until around three kilometres left to race.
"We said in the team meeting I should try for the breakaway and it worked out," Van Garderen said. "I went in the right move and gave it my best shot for the stage win but just came up a little short.
"My condition is definitely good. It’s been a while without racing, and I think for me it was important to get that race rhythm back in my legs. As the race goes on, I am feeling better and better which is a good sign heading towards the Vuelta a España."
Sagan extended his overall lead after his third place earned bonus seconds, his closest rival Dylan Teuns (BMC) caught up behind the crash unscathed.