Belgian rider Jasper Stuyven ambushed the favourites to snatch victory in Saturday's Milan-San Remo one-day classic for the first 'Monument' success of his career.
The Trek rider attacked late and held off the sprinters, with Australian Caleb Ewan second, ahead of last year's winner Wout van Aert of Belgium.
The 28-year-old Stuyven took the biggest win of his career with a well-timed attack with 2.5km to go in the 299km race across north-western Italy.
Ewan led a late sprint ahead of Van Aert under the sunshine towards the finish line on the Via Roma in San Remo, but Stuyven had already taken victory.
"This time, I'm pretty disappointed," said Ewan, the Lotto–Soudal rider already second in 2018 behind Italian Vincenzo Nibali.
"It's always a lottery. I took a risk at the end as that's what I had to do to win. In the end, we waited too long. Hopefully next time."
There was also bitter disappointment for favourites Van Aert, Mathieu van der Poel and Julian Alaphilippe.
The trio had eyed each other throughout the day, especially on the final Poggio climb which usually serves as a springboard for the punchers towards a tense finale.
As expected, Alaphilippe, winner in 2019, went on the attack after some forcing by Ineos, especially Italian Filippo Ganna.
But the French world champion, closely followed by Van Aert, could not widen the gap, unlike the winners in the two previous races, crossing in 16th place.
At the summit of the Poggio, 5.5km from the finish, a group of a dozen riders swung into the lead and tackled a fast-paced descent behind Britain's Tom Pidcock.
Stuyven tried his luck at the bottom of the descent and was joined in the last mile by Team DSM's Soren Kragh Andersen, sitting on the Dane's wheel before launching his sprint from afar.
The former world junior champion has won nine races since turning professional in 2014, but his best 'Monument' finish was fourth in the Paris-Roubaix in 2017.
"There were three very strong riders at the start and we didn't think we would win," said Stuyven.
"But I was having a great day.
"We knew that on the Poggio everyone was waiting for the big explosion (of the peloton).
"I managed to stay in the front group. At the top of the hill, I decided that I would attack and give it all or nothing.
"There were no helpers left in that group, but the big three (Van der Poel, van Aert, Alaphilippe) were there, I knew they would look a little bit at each other.
"The one who closed the gap wasn't going to win. I had to believe in my chances until the end. They were coming from the back.
"It was the hardest final metres of my career, but I've won other races in a similar way.
"It's one of my strengths, to stay out front if they give me the gap. Winning a Monument this way is really nice."
- 'Missed opportunity' -
Slovakian Peter Sagan finished fourth with Van der Poel, among the favourites after his Strade Bianche win this month, in fifth.
Dutchman Van der Poel had dreamed of winning 60 years after his French grandfather Raymond Poulidor.
Sagan was "a little angry to have missed the opportunity" as he finished fourth for the fifth time.
"It's a bittersweet Milan-San Remo for me," said Sagan, who has never won the Italian 'Spring' classic in 11 appearances.
Van Aert also lamented getting it wrong, torn between the desire to attack and save himself for the final sprint in the first 'Monument' of the season.
"I attacked, but not fast enough to make a difference. It was difficult to make the right decision in the end," sighed the 2020 winner.
"Once we hit the bottom, Jasper went on the attack, which was a really good move.
"It was a bit hard for me to chase and a lot of the guys were looking at me. I just came short at the end and Caleb Ewan was a bit faster."