Harry Tanfield became the first Briton to win a stage of the Tour de Yorkshire as the breakaway prevailed in Doncaster.
The 182-kilometre stage from Beverley was virtually pan-flat for the last 90km and looked tailor-made for a sprint field which included the likes of Mark Cavendish.
But the peloton misjudged the day's break and paid the price as Great Ayton's Tanfield used the skills shown in winning time trial silver in the Commonwealth Games last month to take victory on South Parade.
"It still hasn't sunk in," said the 23-year-old, who rides for Canyon Eisberg.
"Coming over the finish line I was like, do I celebrate? I thought I celebrated early, and Alistair (Slater) was going to come back at me. It's just amazing. I never ever dreamt it would happen.
"Everyone was committed, it was a solid day out. I can't believe we pulled it off."
JLT Condor's Slater was second with Mike Cuming of Madison Genesis completing an all-British podium.
Tanfield will start Friday's 149km stage from Barnsley to a summit finish on the Cow and Calf outside Ilkley in the overall leader's blue jersey, but also collected the points leader's green jersey and the grey jersey as the most active rider of the day.
"I was going to re-do my room when I got back anyway so I'll get them all framed up," he said. "It'll look outrageous."
While Tanfield celebrated, others were left to wonder how a six-man breakaway - which never led by much more than five minutes - stayed clear.
"It should have been (an easy catch)," said Ben Swift, racing for Great Britain.
"But when you look at it, there were strong guys up there so full credit to them.
"I don't know if there was panic, it's just the way it was. Maybe we underestimated the strength of the break."
There were no such surprises in the 132km opening stage of the women's race, which did end in a sprint finish won by Dutchwoman Kirsten Wild of the British-registered Wiggle-High 5 squad.
Wild won here in 2016 when the women's Tour was a one-day race, but with it expanding to two days this year she became the first woman to pull on the blue overall leader's jersey.
The 35-year-old pipped Boels-Dolmans' Amalie Dideriksen to victory on the line, with Canyon-SRAM's young British sprinter Alice Barnes third.
Wild put to use her experience with two years ago as cones on a roundabout just over a kilometre from the finish caused confusion in the lead-out trains, sending some riders the wrong way.
"It was a bit chaotic with the signs, but I knew from the map and the time I was here before I had to go straight," Wild said. "So we just went straight, but that put us really early on the front so it was quite hard."
With bonus seconds applied, Wild leads the general classification by four seconds from Dideriksen, with Barnes and Dani Rowe a further two seconds back heading into Friday's 128km stage to that same summit finish on the Cow and Calf.
Former Olympic track champion Rowe, riding for Great Britain, picked up bonus seconds on the only categorised climb of the day and in the intermediate sprints as she rode aggressively early on.
"The plan was pulled off today," Rowe said. "I wanted to try and get some bonus seconds and to test the legs on a climb, even though it doesn't compare to tomorrow's climb."